HK NFA Conversions - Machine Gun Price Guide

7/31/2019 HK NFA Conversions - Machine Gun Price Guide

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The following article is republished with permission ofSmall Arms Reviewand the author, Thomas T. Hoel of Tactical Advantage. Thearticles were originally published as a 3-part series in the April, May and June, 2000 issues of SAR.

The Mechanics of the Heckler & Koch NFA Conversions

By Thomas T. HoelPhotos by James Bardwell, Tom Hoel and Dan Shea

With the Civilian NFA Weapons collecting market in a continual stage of flux, one thing remains constant. Those

firearms of the general type almost universally known as H&K models remain THEmost favored and desirableamongst those collectors of firearms of the modern, post World War II era.

Their almost mythic popularity and desirability are not without pitfalls however, as unless one is well versed inwhat is available in terms of the mechanics and cosmetics of the available versions, it can come as a rudeawakening to discover what one has acquired is not necessarily what one expected! The fact that these guns, asa class, are some of the priciest civilian legal semi-automatics makes it incumbent upon the owner, or individualconsidering ownership, to grasp the many significantfactors that determine relative value of the vastly differentincarnations of these guns. With very few exceptions, those H&K type machine guns available for civilianpurchase as fully transferable Title II NFA weapons all began life as Title I semi-automatics. This the basis forthe most confusion, as there were many different routes taken to convert the guns into functioning selective firefull-automatics. These converted semi-auto guns are often surrounded by confusion and misunderstanding, andit is that problem that this article will address in detail.

Factory HK33E machine gun showing push-pin, swing down typeof trigger group attachment.

Even the common identifying term of H&K is a misnomer since an examination of the history and pedigree ofthis general firearm design will reveal that the salient features of its design were well established during the late

MM23 image by H. Hardin

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WWII years in the work of the famous Mauser-Werke firm on the evolutionary prototype Sturmgewehr-45. Onlyafter a period of considerable movement around continental Europe post-war did this basic design eventuallyfind a permanent home with the reconstituted pre-War Mauser factory in the guise of the then new West Germanfirm of Heckler & Koch GmbH, and its ultimate final conversion to the then new NATO 7.62mm caliber. Thisoccurred, after having been initially designed and experimentally first produced in post-war Spain by theGovernment entity CETME during the period 1949-1956, with the engineering guidance of Ex-Mauser-Werkeemployees who had fled Post-War Germany and settled in Spain. A fter a hurried initial adoption of the new riflein 1957 by both the Spanish and West German militaries, the Spanish and West German Governments co-operated for a time on the final development and perfection of the basic operating system until it became clearthat there existed differing requirements for a new service rifle in each country. Development had begun in

Spain not only because the prominent ex-Mauser-Werke engineers were presently residing there, but alsobecause West Germany had initially adopted the also then new FN FAL rifle as the Gewehr-1, the G1" FAL. Onlywhen it became quite clear that FN-Herstal was not going to allow a production licensing agreement to takeplace, allowing for West German production as opposed to direct contract purchase from FN, (...owing to alingering bitterness over NAZI Germanys roughshod run over Belgium) did the West German Government beginin earnest a search for a suitable equivalent rifle design that could be domestically produced. The differingspecifications extant in Spain and West Germany by 1963 caused termination of the second joint-developmentagreement and ultimately resulted in the production of two similar, but different versions of the perfectedoriginal CETME 7.62mm NATO caliber Modelo C rifle design. The CETME produced versions issued to theSpanish military were in a slightly different form than the now ubiquitous German Army standard Gewehr 3, or

G3" as it became universally known. Parts and fittings were not fully interchangeable, though fundamentallysimilar.

That Heckler & Koch has become synonymous with the particular mechanical design type is due more to

that firms aggressive marketing and promotional strategies, than any original conception of the designparameters. Although since the pre-War Mauser-Werke firm resurfaced as H&K GmbH, there were no doubt amultitude of significant individuals still on hand from the time of its wartime conception, and the return of theoriginal engineering team to West Germany from Spain/CETME no doubt cemented HK GmbH as the preeminentproduction facility of this design weapon. It goes without saying too that H&K GmbH has considerably minedthe potential for expansion within the basic design parameters in the ensuing years, producing a vast family ofmechanically similar weapons. Clearly this potential is owed in large part to the soundness of the originalconcept which allowed an inherent capacity for expansion into differing calibers and applications.

So for purposes of discussion, when we talk about an H&K type firearm we are using that term in the generalsense, an even more important distinction when it is known that there have been more than a dozen countrieswhich have produced guns to the H&K military pattern(s) since the mid-Fifties, with a few even producing gunsin the semi-automatic configuration for intended sale to law enforcement or civilian markets worldwide wheresuch sales were lawful. H&K type guns made by H&K GmbH licensees to the pattern of the semi-automatic

versions made and marketed by the West German firm in the USA have also been imported here in substantialnumbers over the years. Imports of these semi-auto versions were clones of the HK91 .308 caliber rifle andcame from Greece (Springfield Armory SAR-3/SAR 8) and Portugal (FMP XG3S; various importers). H&KGmbH had the sole honor of manufacturing and offering for import through their US based sales and marketingarm, HK USA, Inc., civilian legal sporting gunsin the .233 Remington and 9mm Para calibers with their modelsHK43, HK93, HK94, and their pistol version gun, the SP89.

Foreign Licensee production and importation ofsemi-automatic sporting armswas limited to the .308 caliberclass weapons, the HK91" clones. No COM-BLOC 7.62x39mm M43 caliber sporting weapons were everimported into the USA by anyone, though H&K did catalog a few versions of their guns in that caliber formilitary sales. (NOTE: as this article was being prepared it appeared as though a new US based concern, SpecialWeapons LLC, has obtained ATF approval to begin manufacture and sale of a US made version of the HK91 &HK94 type semi-auto Sporting rifles using receivers manufactured in the USA, with other major componentsobtained from foreign Government Licensee producers of HK type weapons, together with enough USmanufactured minor components to qualify the guns as being Made in the USA according to the new Importrestrictions currently in effect. The information available seems to indicate that these new made receivers will beclose copies of the original design, though not identical in all respects)

I want an HK machine gun!

I want an H&K machine gun, but it has to be ful ly transferable...What are my choices? Exactly two. In orderfor any H&K type machine gun to be classed as fully transferable to individuals under the auspices of theNational Firearms Act, it must have either been imported prior to the enactment of the GCA 68, or else

manufactured here by either a licensed Class II Manufacturer or an individual filing an ATF Form 1 makingapplication, pr ior to the passage of the Volkmer-McClure Firearms Owners Protection Act of 1986.




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Two basic receiver attachment types.

On top, a factory original machine gun with push-pin attachment, and on bottom afactory original semi-auto receiver with the clip on attaching point. The semi-auto

has been modified to fit a flapper style magazine release to emulate the factory MP5.

What this means in reality terms is that almost allof the fully transferable H&K type machine guns available for

purchase by individuals are those guns converted from semi-automatic versions originally imported as Title Iguns, since only a very, very few of the factory original machine guns were ever imported prior to GCA 68. Andthe vast majority of those Pre-68 factory guns being imported were imported either in an attempt at attractingLaw Enforcement and/or Government sales, or for sales directly to those entities, which means they may neverhave moved into the arena of normal commerce. There were also an extremely limited number of H&K weaponsimported into this country Pre-1968 by domestic Firearms companies who sponsored the designs under theirown name and model designations in US trials, and would have built and supported them had the contracts everbeen awarded; the guns from the firm of Harrington & Richardson are most well known, and these gunseventually made it into many private collections when the H&R Museum collection was sold off. It is known,also, that a certain small number of private individuals did in fact personally import these factory original gunswhen it was still legal to do so, so they represent a legitimate source of fully transferable factory guns, thoughtheir numbers are unfortunately extremely small and their collectors value is understandably extremely high. Soif we wish to discount these select few fully transferable factory original machine guns on grounds of pricing orrarity we are left with the vastly more common NFA registered conversions of their semi-automatic cousins,

which shouldindicate our choices are narrowed considerably, but unfortunately that is not the case!

What causes the confusion about these conversion guns is that they can exist in a myriad of differingmechanical and cosmetic configurations, and that within a specific sub-type of conversion method there are evendifferences. All conversions are NOT the same, or equal! Within this framework we will be examining thediffering mechanical methods used to perform the conversion of a semi-automatic weapon into a select-fire nearcopy of the factory machine gun. Previous works have examined the vast catalogs of offerings from those whoactually performed the conversions on these guns when it was a legal enterprise; this treatment will be limited toan examination of the mechanical means by which these conversions reached a means to an end of obtaining anH&K type machine gun available for individual transfer and ownership. Since this is a specific treatment of theconversions of those semi-automatic guns originally imported as Title I Sporters, by nature we will only beconsidering those guns known commonly as HK91 (HK41), HK93 (HK43), HK94, and SP89 types.

Two SEF style plastic trigger pack housings.

On the right is an unaltered factory original pushpin mounting unit, and on the leftis a modified push-pin housing altered to fit a semi-auto clip on receiver mounting.




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(Note the relief cuts to the face of the front metal, and the fake pushpin installed onthe modified unit.)

There were other special versions of the basic rifles imported and sold on a very limited basis by HK USAVersions of the .308 NATO caliber HK91 known as the HK911, SR-9, SR-9T & SR-9TC were imported, as were adramatically small number of a special Marksman's version of the HK94 9mm rifle. Due to import restrictionfactors of when they were imported, these guns were originally priced considerably higher than their standardcounterparts. They were also configured in vastly different styles than the standard rifles, so it is extremelydoubtful if any of these guns were ever used as the host gun for a select-fire conversion, although with the

increasing rarity and escalating prices of pre-Ban guns it is quite possible they may one day be used as thehost weapon for a conversion project using a NFA registered sear or trigger pack. (Due to extreme cost andmechanical differences we will not be examining the PSG-1 Precision Marksmans Rifle, although it too wasimported as a semi-automatic Civilian legal Rifle.)

As far as the non-West German origin imported Sporter r ifles go, there may have been different designations orappellations given to the various clones of the HK91" type rifle by non-H&K GmbH producers, but the guns aremechanically the same and will be considered on a parallel basis to the West German produced HK91.

The Receiver: Where It all Begins.

Machine gun or Semi-automatic, ALL H&K type receivers are the same!Lets take a long look at THATstatement because the receiver of these conversion guns is often the point of the most contention when it comesto understanding, and defining, a particular conversion method, or its official legal status under the Regulations

of the National Firearms Act. And sometimes, more importantly, interpretationsof those Regulations by BATFTechnology Branch.

To understand the above statement in the context of a converted semi-auto Sporter gun into a mechanicallyviable select-fire version, one needs to understand how the receiver component is actually manufactured. AllH&K type receivers are made by high pressure sheet metal press-forming under a mandrel... commonly called

stamping. This finished formed blank, formed in the shape of both sides of the receiver laid out flat, is thencarefully folded and seam-welded into the common finished form or receiver body. Part of the forming processis the punching or indexing of the various holes in the receiver required for assembly, or alignment of the sub-components or assemblies of the completed weapon. Up through this point ALLH&K type receivers, within acommon caliber grouping, are the same during the manufacturing process.

One of these sub-assemblies is the entire fire-control group, which contains the fire-control trigger pack whichis mounted inside the trigger grip housing, and that together are commonly identified as the trigger group on

H&K type guns. (For purposes of this discussion, we need to standardize some terminology regarding the firecontrol components, as even the factory official nomenclature can be confusing at times. The fire control groupas mentioned above is composed of two distinct sub-assemblies: the outer grip frame housing, which can beeither of full sheet metal construction or in later versions of a hard plastic molded over an inner metal frame,and the internal sheet metal trigger pack which contains and provides a secure mounting for the individualmechanical component pieces which provide the fire control functions.) These fire control groups can come inmany different styles as will be examined, but in basic terms they were produced in either of three majorcategories. Originally with a slab-sided full sheet metal exterior housing with a separate contoured palm grippiece offering the standard S-E-F (Safe, Semi-auto, Full-auto) fire control selections, and later a cast metallichousing with a molded in ergonomic finger/palm grip. With the introduction of the full plastic style ergonomicfeatured housings, a wide range of fire control selections was offered including groups with standard S-E-F, 2or 3 shot burst features either alone or with semi-automatic selection. The latest groups are even offering asecond external configuration of full plastic housing resorting back to a smooth grip contour while introducingan ambidextrous control selector feature to the full range of fire modes.

On most factory original machine-gun versions the fire-control group is mounted by, and pivots on in the front,a horizontally sliding assembly pin front and rear. (Some notable factory original exceptions to this trigger groupmounting rule are machine guns such as the HK21E and G41, which for specific and intended design purposes,are equipped with a non-pivot pin type front trigger group attaching mount. A contrary exception to the generalrule is the MSG90 Selected Marksmans Rifle; though the MSG90 is intended as a semi-automatic weapon it isdesigned with a front pivot-pin mounting attachment for the semi-automatic fire control group, and as such,BATF has classified this particular weapon as being legally classed as a Machine Gun even though it isincapable of automatic fire in its intended form.) This front assembly/pivot pin is aligned and held by a set ofindexed holes located on a rounded protrusion of the receiver bottom at the rear of the magazine housing upperarea. It is mated with a permanently affixed (usually welded, though press fitting is sometimes used) fittedbushing on the inside of the two pressed steel sides of the receiver stamping for the assembly/pivot pin to passthrough and bear against. By this method the trigger housing can be held fast in the proper position on thereceiver, and also pivot downward for disassembly or cleaning. This arrangement is commonly known as the

push-pin trigger group assembly, or swing-down trigger housing type. It is the original and most commonmethod of attachment of the trigger housing on factory machine-guns. This feature alone, the holes and matedbushing for the front assembly/pivot pin, has been determined to be the sole defining feature by BATFTechnology Branch as to what constitutes in a legally defining sense a Machine Gun receiver for a H&K typeweapon under the National Firearms Act.




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As a general rule these types of conversions retain the single legally defining feature of the clip-on triggerhousing attachment, even if the receiver itself was NFA registered as the machine gun in the case of thoseconversions employing an unregistered conversion part. So if the receiver itself was registered as the NFAcontrolled item, but not mechanically converted at the time of conversion to utilize a swing down lower, themechanical alterations to achieve semi-automatic fire capability must have been done to other components ofthe weapon in question.

Two clip-on style housings.

On the right, a factory machine gun housing is adapted to fit a clip-onstyle mount. On the left is an unaltered, original semi-auto housing.

Such is the case that the mechanical differences between a semi-auto-only capable rifle and a fully automaticcapable rifle can be contained completely within the fire control elements located in the trigger group. It iscommon to employ the phraseology conversion part for a single NFA registered piece such as a registered HK

Auto Sear when used as the basis of a lawful conversion. And the phraseology of conversion device is used indescribing an assembly converted as a unitized item utilized as the basis for lawful conversion, such as theadapted factory select fire trigger packs and grip frames. So when we speak of an unregistered conversion partor device, or a NFA registered conversion part or device, we are relating the fact that within the fire controltrigger pack, certain parts not present in the semi-auto-only capable version can be introduced to replicate thefunctionality of the factory original selective fire version, all without mechanically altering the actual receiver of

the gun. This is the case whether we are discussing a Registered Receiver conversion where the actual receiverof the gun retains all the defining semi-auto features, yet is NFA registered itself, or if a Registered ConversionPart is utilized to convert an existing Title I gun. In both of these legally disparate cases, the receiver of the gunremains unaltered mechanically. All mechanical changes necessary to affect conversion into a functioningselective fire weapon are carried out on the fire control group alone.

Whether registered or unregistered themselves as conversion parts, the introduction of these additional partsinto a semi-auto fire control pack is designed to restore the three missing functions present in the selective fireversion. First, since these guns fire from the closed bolt and are hammer fired, there must be a mechanicalmeans to prevent hammer fall until the bolt is locked into battery. Secondly, there must be a means to time thathammer fall correctly. And lastly, there must be a means to select the mode of fire desired. In H&K style gunsthese three requirements are met by the addition of two parts not normally present in the semi-automatic-onlyfire control pack.

The first and most important of these missing parts is what is known in official H&K nomenclature as aCatch, but is more commonly understood as an automatic sear; this part serves to hold the hammer fromfalling until mechanically released at the proper time. It is also present to prevent inadvertent or prematurehammer fall, in short a secondary safety system. H&K nomenclature specifies that the part performing thedisconnector function is known officially as the sear, because it functions in both capacities when it is operatingindependent of the automatic catch. The automatic catch and sear are totally independent mechanicalsystems, operating in conjunction only with the selector, or change lever in factory parlance. With the selectorlever in the cyclic fire mode, these two independent systems operate in parallel to allow cyclic fire operation. Theautomatic catch ALWAYS functions, only when the selector lever is set on cyclic mode does it allow the trigger tolower the sear out of position to prevent the disconnect function. This means the only mechanical system actingon the hammer then is the automatic catch, functioning as the primary sear in this mode. This is why it hascome to be commonly known as the automatic sear. (To avoid further confusion we will use the term sear orautomatic sear when specifying the factory term of catch, and disconnector in place of the factory term sear.This is due to the universal application of these terms currently in the Civilian NFA community, not to instillconfusion with factory nomenclature!!)

The only other required additional missing part is what is called the release lever. This is a vertically mountedswinging arm located on the forward right side of the trigger pack frame. At its mid-section it is in contact withthe extension arm of the catch(auto sear). Its upper end is located in the path of travel of the right undersideof the bolt carrier which activates its forward movement upon final dwell of the bolt carrier after the bolt has




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moved into battery, causing release of the catch and hammer fall.

Two semi-auto clip-on mounting style trigger packs.

On the right, an adapted factory semi auto trigger pack frame with registeredsear installed. On the left is an altered factory machine gun pack.

In terms of component parts, these additional two parts are all that distinguishes the two versions of the basicfire control pack, although the actual physical specifications of certain parts vary slightly between the twoversions. With these simple additions to the standard semi-auto trigger pack, and minor modifications to a fewother small areas discussed below, a functional selective fire conversion can be achieved. The fire controlselector switch is identical in both semi-auto and selective fire variants of the guns, and is merely preventedfrom moving into the full-automatic selected position in the semi-auto trigger groups by a mechanical detentformed into the exterior of the trigger housing.

In real life terms, these so called conversion parts usually took the form of a proprietary design of the catch,or auto sear, although there were a number of variations on that theme. Besides several variations of autosears, such other items will be encountered as combination release lever/auto sears, or combined auto sear andpivot. In those cases, only the catch was legally defined to be the actual conversion part; the release lever

and required modifications to the trigger frame itself to affect proper installation were not considered in a legalsense. In most installationsa factory release lever was utilized without modifications.

When moving into the realm ofNFA registered conversion parts or devices, it was mechanically almost thevery same items and choices, the single legally defined conversion part was simply marked and registered as the

machine gun, it being the only control led part. When exploring NFA registered conversion items thoughanother category is sometimes encountered, that being the registered conversion trigger packitself, as acomplete entity. This was the actual stamped metal trigger frame of a stock factory produced MG trigger groupcomplete with all unaltered factory MG parts, suitably modified along with its companion trigger housing to fitthe clip-on attachment of a semi-auto receiver, marked and registered as a complete unit, and were known tohave been NFA registered as a conversion device. While rare, these units are highly desirable as will be shown.

(Just to muddy the waters a bit, there are known a very few Title I guns hat have had the receiver triggerhousing attachment pointoriginally converted to a push-pin, swing-down style while employing a

Registered Auto Sear as the NFA registered item, leaving the now highly modified Title I receiver in aquestionable legal status. These use a modified semi-auto trigger pack the same as most Registered Searconversions, and ATF is surely not happy with these guns for sneaking through, and their legal status is at bestmurky.

Additionally, some of the very first HK91" style semi-automatic rifles imported were actually made with areceiver that was identical to a factory MG, that is it used a push-pin type trigger housing attachment to attach aSEMI-AUTO capable only trigger mechanism, which was nothing more than a select fire trigger pack modified toprevent automatic fire. These guns were even identified on the receiver as G3", and were imported by Santa FeDiv., Golden State Arms Corp. A select few were snapped up by enterprising Class IIs and some enlightenedindividuals and converted to transferable machine guns by registering the actual receiver and simply pinning ona standard factory MG trigger group. Such a gun would have doubly interesting value to a collector!)

It can be readily seen then that the most desirable method of conversion was just to fully restore the receiver of

a semi-automatic gun back into a machine gun receiver able to use factory original machine gun, push-pinattachment type, swing down trigger groups by removing the clip-on lower attachment block and preparing theforward attachment point to accept a pivot-pin attachment. This was most usually accomplished by milling offthe clip-on ledge, and then align drilling the two holes in the lower rear of the receiver, followed by welding inthe pivot-pin bushing. With these modifications accomplished a factory swing down trigger group could beattached in the normal manner, and that combined with installing a factory original bolt carrier (or amodification to the semi-auto version) completed the mechanical alterations needed to produce selective fire. If




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this method was chosen, at the same time a factory machine gun type flapper style magazine release couldalso be installed in the correct manner, mimicking fully the factory machine guns.

So why werent al lthese guns being converted this way?? Two simple reasons. First, with the surplus partsmarket the way it is today many people are unaware that things werent always as they are now. The availabilityof foreign machine gun parts and/or parts kits in the quantities they are seen now is a fairly recent phenomenon,due to a positive change in the relevant laws. The ready availability of these parts today from factory producedmachine guns makes it much easier to contemplate doing a near exact copy of the factory made gun as many ofthe needed parts could be simply exchanged. This situation however was definitely notthe case during the timeframe that most of these semi-automatic guns were being converted and registered as Civilian transferablemachine guns. The most common reason that so many Class II manufacturers performed conversions usingheavily modified semi-auto parts is that they had no choice! The registering of the semi-auto configuredreceiver, as the machine gun was just the most commonly accepted sequence, after which one went to work onthe semi-auto trigger group parts to affect the needed mechanical changes to render automatic fire. Then afurther realization, due to a late ATF ruling, that certain dedicated parts used to convert a semi-auto gun toautomatic fire mode were also now being legally classedas machine gun conversion parts in and of themselvesprompted a rush to register these parts alone as machine guns in large quantities. It became far moreeconomic in terms of time and manufacturing ability to concentrate on producing these so called conversionparts then it did to take the time to locate, obtain, and mechanically convert complete guns. Such was thegenesis of the common Registered H&K Auto Sear. Thus a time line developed whereby beforethis ATF rulingmost NFA conversions had the actual receiver of the gun registered as the machine gun, whether or not thereceiver had actually been fully restored into a machine gun receiver, and after the ruling most manufacturersresorted to registering the conversion part, or device, or kit. It has become synonymous within the industry torecognize the term Conversion part as exclusively meaning a conversion auto sear (the catch in HK

terminology), whether it is in and of itselfNFA registered or not. This is due to the BATF determination andlegal classification of this part as a machine gun as described above. As will be seen, the other required partsfor a select fire conversion are not discussed separately in a legal definition.

It was a lucky Class II manufacturer who was able to procure enough original and authentic factory mademachine gun parts to perform a complete receiver conversion back into the factory select fire specifications.Those authentic parts that were available were always in short supply and priced accordingly. With currentimport law, price trends and world market forces, many do not remember that the cost of a conversion job to asemi-auto gun was considered prohibitive if it approached a few hundred dollars!! Thus, factory original partswere often viewed as a serious cost factor. Modifying the original semi-auto configured parts was often done asa matter of course, and economic reality. With H&K type guns it was always parts for the .308 caliber G3 riflesthat were in greatest supply and so authentic parts, for say an MP5 SMG, were often nearly impossible to locatewith any consistency or price control. This is one reason why there are such a disproportionately low number ofregistered receiver conversions that were done by a complete restoration to the factory select fire push-pin type,

swing down trigger group, specifications.

The other major reason for registered receiver conversions being performed with unregisteredconversion partsemployed on a NFA registered unaltered semi-auto receiver was due to the simple fact that many of the activeClass II manufacturers in the heyday of the conversion craze were less than Journeyman level skilled machinistsor Gunsmiths. Compared to the recognized handful of true Artisans who today are active in the H&Krepair/modification arena, there were some shops that turned out vast numbers of conversion guns with less-than-perfect machine work on them, and as a consequence many of these same shops quickly realized it was aloteasier to leave the expensive receiver alone and perform the needed mechanical alterations to the more easilyworked on fire control packs and trigger housings. The required alterations to the semi-auto bolt carriers ofguns from this general pedigree for instance, are often times of dubious quality and in need of rework to achievereliable functioning.

It is fact that many Class II Manufacturers, and Form 1 approved individuals, decided to employ an altered semi-

auto trigger pack for ease of conversion, while registering the receiver itself as the NFA controlled item. This ofcourse left a dilemma as it allowed the existence of a semi-automatic configuration receiver which was registeredas a legal machine gun, while producing an unregisteredconversion part, which by law was allowed to exist aslong as it alwaysexisted in physical contact with the NFA registered semi-automatic configuration receiver.

Into the murky depths...

ATF Technology Branch has quite emphatically ruled that a registered receiver gun, that as a registeredconversion using a clip-on style trigger group housing, cannot be later converted to a different method of triggerhousing attachment. In other words, a registered receiver conversion originally mechanically convertedwith aclip-on style trigger pack attachment method cannot later undergo a further alteration to a push-pin, swingdown lower attachment provision. Plainly stated, ATF has ruled that the removal of the clip-on ledge piece ofmetal and subsequent drilling of the two pivot pin holes is, in and of itself, the act of manufacturing a NEWMACHINE GUN, regardless of whether or not the actual receiver is already registeredas a machine gun!! While

this may seem to make no logical sense at first glance, one must understand why ATF has taken this positionand that is because they know that a registered receiver converted gun employing a clip-on style triggerhousing will have had to have been mechanically convertedusing a semi-auto fire control pack that has beenmodified to function in the selective fire mode, or in very rare cases a factory original machine gun pack andtrigger frame altered to fit the clip-on attachment of the semi-auto receiver. They know that if you remove theclip-on ledge from the receiver, post initial conversion date, that you can then use a normal factory original




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machine gun-style fire control packall of which leaves the very interestingquestion of what happened to theoriginal converted semi-auto fire control pack which had been modified to fire in the selective fire mode (or amodified machine gun pack and housing) and could, in and of itself, be used to convert another semi-auto gun!!Because the legal definition of what constitutes a machine gun also defines conversion parts, or assemblies, tobe machine gunsin and of themselves this leaves an extra unregistered machine gun out there. This is whythe subsequent removal of the clip-on ledge, post initial conversion date, is creation of a NEWmachine gunbecause presumably it frees up an unregistered conversion part (or assembly)! With a clip-on style triggerhousing attachment method the actual NFA registered machine gun, as it was originally registered, is legallydefined to be the combinationof the semi-auto receiver and the original unregistered conversion part...not justthe receiver alone!

Two converted selective fire packs:

On top, a converted pack utilizing a registered sear, and on the bottom, is a factorymachine gun pack adapted to fit a clip on mounting. Clearly visible are the differentlocations of the sear and pivot pin hole. The differing geometries are what cause a

difference in the cyclic rate of fire.

The genesis of this ATF philosophy is derived from the enactment of the Hughes machine gun freezeamendment which was slyly inserted into the otherwise good natured McClure-Volkmer Firearms OwnersProtection Act, enacted on that now famous date May 19, 1986. The Hughes amendment forever capped thenumber of fully transferable machine guns available to civilians, and this fact is what drives the ATF position onfurther modification of already registered receivers. Even more interesting in this case was the position ATFTechnology Branch took on the status of literally thousands of lawfully NFA registeredmachine gun conversionparts or kits, most of which were hastily registered shortly before the enactment date of the McClure-Volkmer

Act. ATF Technology Branch had initially taken the official position that any lawfully registered machine gunconversion part or kit could be installed in the host firearm at a date later than the initial registration date. Thismeant that a manufacturer or lawful registrant could hold onto that registered conversion part or kit indefinitelyuntil it was desired to affect the actual installation into a host firearm. Problems soon arose however when ATFField Agents tasked with checking on the compliance of Class II manufacturers soon discovered that certain of

these manufacturers, in a greedy rush to register as many fully transferable conversion items as possible prior tothe cut-off date, had been less than forthright in their descriptions of the conversion parts or kits.

Many of these parts or kits fully failed ATFs basic test for what constitutes a machine gun conversion part ableto be lawfully NFA registered, in and of itself, as a machine gun. ATF Technology Branch had had to make thedecision to allow the classification of certain items as machine gun conversion parts or kits based upon thetechnical possibility that installation of one of these parts of kits, without modification to the actual firearmreceiver itself, could affect the alteration of the firearm into a fully automatic mode of fire. This was historicallypresented once before by the Technology Branch ruling in early 1969 on the NFA status of the so-called M2Carbine Conversion kit after the enactment of the Gun Control Act. The key element in the allow ability of a

conversion part or kit was that it had to be able to be installed without any additional modification to thereceiver of the host firearm. Any amount of modification was allowed to any othercomponent parts, but not tothe actual Title I receiver or component parts defined as the Title I receiver. Unfortunately, many unscrupulousClass IIs, and some Form 1 approved individuals, had simply registered parts or kits that could in no way be

physically installed without further modifications to the host firearms receiver. The now classic registered AKauto sears are the best example, although all manner of things were attempted including dead-stock M16 autosears for drop-in installation into AR15 type receivers!! These conversion parts or kits were in no wayphysically possible of being installed without significantalterations to the host firearm receiver, and ATF soonhad a major dilemma on its hands since literally thousands of these NFA registered conversion parts or kits nowexisted and they had to go about defining which ones were lawfully registered, and which ones were




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erroneously registered. There was also the significant question as to the status of those host guns which hadpreviously had such parts or kits installed. Between May 1986 and December 1987 ATF had grudgingly allowedthe installation of these NFA registered conversion parts or kits requiring additional receiver modifications,insisting that any host weapon so converted into a machine gun have the registered conversion part or kit

married to the host receiver by serial number on the relevant registration form effectively forever preventingthe future separation of host receiver (now modified) and the registered conversion part(s). This was to preventsubsequent removal of the registeredconversion part, freeing up a modified Title I receiver which could acceptnew parts and continue to function as a machine gun. This concept of future separation is key to understandingthe H&K conversion dilemma!!

Two clip-on style housings.

On top is a semi-auto pack to fit a clip-on style mount. On the bottomis an altered FBI S-F pack frame which was originally configured thesame as a machine gun pack. The FBI S-F ambidextrous packs are

very uncommon and command a premium price.

At the Las Vegas SHOT Show in January 1988, ATF declared in their Industry Seminar that those T itle I semi-

auto guns converted using a registered machine gun conversion kit, but which had required receivermodifications to be contraband and illegal!They further stated that those guns already convertedusing suchmethods would be considered individually, but that no further such usage of those type of NFA registeredconversion parts or kits would be allowed. In effect, anyone holding a NFA registered machine gun conversionpart or kit, that required receiver modifications to install it but had not already performed such an installation,were now in possession of contraband. This ruling was quickly applied universally to other conversions of semi-auto guns that had had their unmodifiedTitle I receivers NFA registered as machine guns, but had beenmechanically converted to automatic fire by the use ofunregistered conversion parts, a practice heretoforehaving been declared marginally acceptable. The H&K conversion story all of a sudden got considerably lessclear at this point. According to ATF, what now existed with H&K type NFA conversion guns were severaldisparate legally defined classes of registered conversions, though ostensibly all on the same type of gun!

So...just whatAREyou looking at? The many possible scenarios.

First, you had those Registered Receiver conversions that had been originally mechanically convertedto a legallydefined machine gun receiver employing a factory MG style push-pin, swing-down style trigger housing. Theseguns occupy the pinnacle position of NFA conversions desirability status due to their unquestionable registrationlegality, duplication of factory mechanical features and cosmetic appearance, and ability to allow owner changesin factory style trigger groups along with enhanced spare parts availability.




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Three styles of selective fire packs.

On top is a converted pack using a registered sear, thecenter unit is a converted original factory machine gun

pack adapted to fit a clip-on mounting. The bottom is anunaltered original SEF pack for push-pin mounting.

Next up, those Registered Receiver guns that had the unmodified Semi-auto Title I configuredreceiver NFAregistered as the machine gun, but had been originally converted with an unregistered conversion part ordevice. These guns have several handicaps and vices. First, because of the above mentioned legal rulings theirreceivers could never be further altered to a push-pin, swing-down trigger group mounting style,

which committed them to forever using the same trigger group and fire control pack as had originally beeninstalled on the date of initial conversion and NFA registration, with one minor possible exception. Because ofthe usage of that unregistered conversion part in the modified semi-automatic fire control pack, that conversionpart itself could never be replaced, only repaired!This is legally prevented because to replace it with a newone, if damaged or lost, would constitute new manufacture of an unregistered machine gun which was madeillegal by the new 1986 Laws. Also, this prevented easy replacement of trigger group spare parts, as to convertthe semi-automatic fire control pack to replicate selective fire, many of the fire control parts had to be heavilymodified themselves. This was due to an altered mechanical geometry required by the need to have the samemechanical functioning as a factory selective fire pack, but in a geometry necessitated by the requirement ofhaving the lower front edge of the fire control pack adapted to mount on the clip-on ledge of the semi-autoreceiver. So, to get suitable replacement or spare fire control parts, one has to have standard parts that werespecifically altered for usage in such a conversion, which are not typically available as easily as factory MG partsare now. The key is that with these types of Registered Receiver conversions, the NFA registered machinegun is the original combinationof the unaltered clip-on style receiver, and the originally installed unregistered

conversion part...those two entities can never legally separate, they are married for the life of the weapon!! Allof these reasons are why this type of Registered Receiver conversion occupies the lowest level of desirabilityamongst registered receiver conversions, and are priced accordingly.

Even more disheartening in present times is the fact that since conversions of this type use an unregisteredconversion part of most likely a proprietary design, unless you can re-install that exact conversion part ina different pack, you cannot replace the original style fire control packwith a perhaps more desirableversion such a Navy Group, or 2-or-3-shot Burst style ambidextrous lower group!! (This problem also effectsconversions using a NFA Registered Sear as will be seen)

[LEGAL WARNING! While it may be physically possible to adapt a factory select-fire ambidextrous style NavyGroup, or Burst type trigger pack and plastic housing to fit a clip-on semi-auto receiver, it is patentlyILLEGALto simply adapt such a trigger group to fit, and then just clip it onto a Registered Receiver gun usinga clip-on style housing attachment as you have just made a new, unregistered machine gunby doing soyouve

just created a complete conversion deviceto convert a semi-auto receiver!! The legal issue here, again, is thatyou are left with the original, unregisteredconversion part being freed up when you swapped them out on yourRegistered Receiver conversion. There are currently available many of these adapted

Navy/burst/Ambidextrous trigger groups offered for sale, but they are ALL illegal, unregistered conversiondevices!! The ONLY way they are BATF legal is if they have been NFA registered as a Conversion device! Thishad to be either before the May 19, 1986 cut-off (which is HIGHLY unlikely due to very limited availability of




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these styles of factory parts then), or as a post-86 Dealer Sample Conversion device, or made at the request ofLaw Enforcement Agencies. ]

Lastly, somewhere in between the above two more common versions of registered receiver conversions are avery select group of Title I guns that were converted to select fire by a more questionable method...at leastaccording to ATF Technology Branch after their early 1988 determinations and rulings. A s alluded to previously,a select fewsemi-auto guns are known to have been converted by having their unmodified clip-on style receiversNFA registered as a machine gun, and then proceeding to attach a normal factory original MG select-fire, firecontrol packand housing that had bothbeen suitably modified together to clip-on attachment, as describedabove. These guns most likely were never visually inspected by ATF Technology Branch for suitability andconformance to approved methods prior to registration, and therefore sneaked by and into the NFRTR. In anyera, these style conversions would be suspect as they contain an unmodified Title I receiver with its clip-ontrigger group attachment point, and an unregistereddrop-on conversion assembly. These guns are ripe for ATFmaking a determination that they were improperly registered, and may be subject to reclassification. Why theoriginal registrant did not originally just register the converted trigger group assembly alone, instead of thereceiver of the Title I gun, is a mystery...and a lingering problem for the current owner. These guns are just toomuch in a legal grey area to attach a reasonable sense of value to, and they are the perfect example of why aprospective purchaser of a registered receiver H&K conversion gun needs to extremely vigilant and aware of

just exactly what particular gun he is looking at.

Within this understanding lies the major financial difference in perceived value of registered receiverconversion guns. A fully converted gun able to use factory push-pin style, swing-down trigger groups is alwaysdeemed to be more inherently valuable then a gun using a clip-on trigger group, though the mechanicalfunctioning is identical between the two guns. The perception is two-fold. First, on receivers able to use swing-

down groups the ability to choose between the various factory offered styles of fire-control set-ups is greater,and they are simpler to exchange. Secondly, on registered receiver guns that employ a clip-on style group, atleast oneof the fire-control parts will be an unregistered conversion part, which means in practical terms thatshould this part break or be damaged it can only be repaired and never replaced, which limits to some smallextent its serviceability. Thus, the spare-parts question of a gun using a registered receiver employing a swing-down style trigger group is considerably better; hence an appreciable rise in value and pricing is accorded.

Above, an extremelyrare 0-1-F-3 style burst group with burst mechanism locatedin the rear of the pack. These are the only burst packs that can mount an unalteredregistered sear. These groups command a substantial price. Below, the inside view

of the 0-1-F-3 burst pack showing a radically different mechanical arrangementcompared with the more common "ambidextrous" style burst packs.

It is easy to see then that a Registered Receiver Conversion is not so simply defined as just a receiver convertedto be functionally identical to a factory original machine gun receiver with a push-pin style, swing-down lower. It




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can exist in different forms, and those different forms determine a large difference in a monetary value, userfriendliness, and perceived desirability.

The Full Registered Receiver Conversion...Top to Bottom

As previewed earlier the most desirable conversion method was to completely restore the Semi-auto Sporterreceiver configuration back into a factory machine gun specification. This actually was done by severalestablished shops and some individuals with the proper know how and skills, when they had the factory originalselect-fire trigger packs to install as part of the job. In order for this type of conversion to be unquestionablylegal it had to have, at the time of initial conversion, the dedicated semi-auto receiver features completelyremoved, and then the two distinctive factory original machine gun receiver features restored. In proper order,

this meant milling off the semi-auto receiver clip-on ledge, then align drilling the two pivot pin holes, andfinally, welding in the pivot pin bushing,ALL THREE STEPS!

In actual practice, this was done with varying degrees of success, and/or completeness. One of the more wellknown Class 2 manufacturers had the habit of roughly milling off the clip-on ledge and drilling two holes, butnot installing the pivot pin bushingwhich leads to oblongation of the pivot pin receiver holes after time. Manytimes the location for the pivot pin holes was roughly estimated by placing a factory style swing down lower onthe receiver and using it as a template for drilling the pivot holes, which obviously led to a great deal ofinaccuracy in their locations, all of which should be verified if a particular gun displays any untoward habits.Those conversions done by competent machinists using factory drawings and/or production data areunquestionably more desirable. Additionally, if a factory MG style flapper magazine release assembly was to beinstalled, the proper alignment of the pivot pin holes and the relative alignment of the pin bushing becamecritical, as this is what the flapper sleeve rides on, and bears against. An improperly aligned pivot bushing canprevent proper magazine release functioning.

Once the conversion of the trigger housing attachment was completed the only other significant detail to properselective fire mechanical function was the bolt carrier. All HK type bolt carriers are manufactured the same, tomachine gun specifications, until they are destined for a particular product line. For use in a semi-auto Sportergun, the under surface of the carrier which contains a milled contour to activate the automatic sear trip lever inmachine guns, is milled straight through, rendering it incapable of use in a machine gun. A restoration of thiscarefully contoured tripping surface is required. The bolt carrier is manufactured from high-tensile strength alloysteel which has been heat treated to resist wear, and on some versions additionally surface hardened in criticalwear regions. This high quality material content and manufacturing method resulted in a piece that wasextremely difficult to modify or alter with ease. To do it properly, the carrier should have been fully annealed,then had the milled areas filled in with a similar alloy content weld or plasma fill, finish contoured to factorysurface spec and then final heat treated, and/or surface hardened, to restore the original wear and longevityproperties. Almost no one ever did this, as it is complex and time consuming, not to mention costly. Usually a

weld fill and regrind sufficed, although a couple shops adopted Stellite welding in an attempt to add wearresistance back into the tripping surface(s). The modified bolt carrier is one of the most commonly encounteredareas of trouble from the original conversions, and fortunately now, a simple solution is at hand. With thecurrent easy availability of original factory produced machine gun parts and parts sets, the best solution is toinstall a factory machine gun style bolt carrier, which has the proper sear trip surface intact.

With these modifications complete, the guns receiver has been fully restored into a virtual clone of the factoryproduced versions. As can be immediately seen, a gun so restored has the advantage of using the full range of

off the shelf factory MG accessory trigger groups and spare parts. With the current ease of obtaining the moremodern styles of plastic housing Ambidextrous trigger groups such as the so called Navy Group, or thevarious Burst style groups, a Complete Conversion is often valued an order of magnitude higher than theunmodified Registered Receiver conversions due simply to its unique ability to utilize all factory parts andaccessories as originally intended without fitting or alterations. Essentially, these guns beat the MG ImportRestrictions contained within the Gun Control Act of 68, and at a considerably lower cost than a true Original

which is undoubtedly now considered a collectors piece.

Second Choice: Conversion Parts and Devices...Registered and Unregistered!!

As we have seen above, unless the receiver of the gun was restored into full machine gun configuration, themechanical alterations needed to produce automatic fire were necessarily contained within the fire-control groupand these modifications were usually performed in a similar manner, whether or not the conversion part ordevice was NFA registered or unregistered in and of itself. In the same fashion as the Registered Receivercategorizations, Conversion Parts or Devices can be divided up into two distinct sub-categories. Firstly, themechanical conversionmay be accomplished by use of a single conversion device, such as a conversion SEAR,or secondly, by alteration of the whole fire control group as a unitized assembly, such as a converted factoryselect-fire pack.

There is necessarily some cross-over and blurring of the lines of distinction in discussing these items as they all

function absolutely identically, the only difference being with which basic components you started, and in whichdirection you preceded to arrive at the same mechanical destination. Conversion partssuch as sears arenecessarily part of conversion groups (devices), they are just discussed as a separate component part asopposed to the unitized assemblies of converted factory original groups, because of the needed mechanicalalterations to install them into functional automatic-fire status in a semi-auto fire control group into which theyare being adapted.




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First off...Registered Conversion Groups.

This category can be loosely described as either Converted Semi-Automatic only fire control groups, orConverted factory original Selective Firecontrol groups Each method has its advantages and limitations.

The equivalent to the Full Registered Receiver Restoration in terms of mechanical purity and factory featurereplication would be what is known as a converted Selective-Fire Trigger Group. As the name implies, this isnothing more than a stock factory produced select-fire trigger group suitably modified to mount onto anunmodifiedclip-on style Title I receiver. These are almost alwaysfound to be an original metal grip framed S-E-F style housing, and an NFA registered item as a conversion DEVICE, in and of themselves, in order to not bein any legally grey area. Al though as mentioned above a few may be found mounted on an NFA Registered, but

unmodified, clip-on style receiver as an unregisteredconversion device. All internal mechanical components arefactory select-fire design and this greatly enhances the spare parts scenario, as there are no modificationsdoneto any of the trigger group functional parts.

When we discuss this specific type of conversion device, we are considering only the actual mechanical piecesrequired to adapt the complete group as an assembly onto an unmodified Title I receiver mounting. This isaccomplished by first adapting the factory select-fire sheet metal trigger pack frame to mount via a clip-on style.Then by adapting the outer frame grip housing to allow for this new mounting method by relieving the lowerfrontal area of the housing box to permit the receiver ledge to protrude through, and finally by providing ahorizontal mounting point for the receiver ledge to hold fast to. This adaptation will also be seen when weexamine the installation of newer style plastic grip frame housings to clip-on style receivers when employingregistered auto sears in converted semi-auto trigger packs. Or in rarer cases, when an owner desires hisconverted semi-auto trigger pack utilizing an unregistered conversion partto be reinstalled in one of these newerhousings. There are significantlimitations to this in legal terms though as discussed above. The NFA registrationreflects the status that the ENTIREunit is the registered item as it requires extensive modifications to both thegrip frame and the enclosed trigger pack to adapt them to fit and function on a clip-on style Title I receiver.These two individual components cannot ever be separated legally, as again, it is the combination which wasoriginally registered as the device. What this means is that exchanging the metal S-E-F outer grip framehousing for a newer style plastic S-E-F grip frame housing is not allowed if the metal housing was part of aregistered combination. Most of these units were marked on the metal housing exterior with the requiredidentifying markings when registered.

The real intrinsic value of owning one of these converted, adapted, and NFA registered factory select-fire triggergroups is obvious in that it is, in a mechanical sense, it is absolutely genuine and identical to those triggergroups that came on factory machine guns, only the mounting method has been altered. The internalmechanical components are fully interchangeable, and more importantly, they are situated in the normal designpositions providing the intended mechanical geometry of function. The designed in mechanical geometry

remains fully unalteredwhich will retain the designed cyclic rate-of-fire, as on a factory original machine gun.This is a most significant point to be understood, as this mechanical geometry is not the samein conversionsdone by altering a semi-auto trigger pack to function in the selective fire mode using a Conversion part, or

registered auto sear. This last statement directly leads us into the most commonly misunderstood arena of thewhole H&K Conversion dilemma...

Sears, Trips, andCatches...the Pandoras Box of Conversion Parts

When HK GmbH set about to redesign the original select-fire control group to mechanically remove theautomatic or cyclic fire mode, they chose to alter one of the original safety features built in to the design. Likethe FN FAL mechanics, the automatic sear is also a form of safety sear designed to enhance the mechanicalsafety by preventing hammer release and firing pin contact before bolt lock-up in the final battery position.Removing the automatic sear removed one level of mechanical safety, as it is always functional whether or notthe fire control selector is in the cyclic fire mode. Selection of cyclic fire mode merely allows the trigger to drop

the secondary sear (also functioning as the disconnector) backward and down out of contact with the hammerengagement surface.

To adapt the currently produced parts with as little change as possible to a Semi-automatic only configurationwas quite easily done from an Engineering standpoint. This adaptation was accomplished with a minimalredesign of the three elements of the fire control group. The main difference between the two fire controloptions lay in the two independent safety systems. Since these two independent functions were arranged asseparate mechanical pieces, they only had to remove the automatic sear (catch) and its attendant release leverfrom the fire control pack to eliminate the option of cyclic fire. With a small modification to prevent the selectorlever from rotating all the way down into the cyclic fire mode position the trigger pack was now arranged tooffer only two positions: Safe and Fire(semi-auto only). The secondary sear (disconnector) remained asalways, merely prevented from dropping down out the way of the hammers travel, thereby always forcing it tocatch the hammer, disconnecting it from the trigger for normal semi-automatic operation. On HK GmbHproduced semi-automatic guns, the primary sear notch on the hammer was also ground off to prevent easy re-

conversions. On some of the licensed produced HK91 clones the hammers are not fully adapted to semi-automatic only operation. The selector levers are identical between select-fire and semi-auto only versions.

Combined with the obvious redesign of the relevant fire control parts to eliminate the selective fire capability andrestrict it to only semi-automatic fire, the actual sheet metal box frame that holds these components wasmodified into a semi-auto-only configuration. With the deletion of the automatic sear and its pivot pin position,the select fire tri er ack frame could be altered to mount on the newl desi ned cli -on receiver mountin




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ledge. This was accomplished by cutting a distinctive notch in the lower frontal area of the sheet metal box, thearea formerly occupied by the sear pivot pin holes. Importantly, one other major mechanical item wasconnected to this location. The trigger (and auto sear) return spring was mounted inside the automatic seararms and was held in place by what is known as a distance sleeve, which is nothing more than a hollow tubecontoured to hold the spring in position and it slips around the sear pivot pin. This single fact is the leastunderstood problem of conversion packs using the clip-on mounting style.

Due to the deletion of the full-automatic sear position in the semi-auto pack, necessitated by the new front clip-on mounting notch, the pivot point for the trigger return spring had to be relocated to a more rearwardposition in the pack frame than in the selective fire frame. This relocation is the major mechanical distinction ofthe registered/unregistered auto sear conversion. The conversion sear is forced to use this new triggerreturn spring pivot pin location as the mounting point for the newly installed conversion auto sear which placesthe conversion sear in closer physical proximity to the sear contact bearing surface of the hammer. The triggerreturn spring used in semi-auto packs is also formed differently than the selective fire version. This relocatedspring must also now function on the conversion sear in the same manner as the factory select-fire unit, but it isforced into a differing contact geometry and so acts accordingly. Whether NFA Registered or not, all these

conversion sears mount and function the same! The onlysignificant difference is found to be whether or notthe original factory release lever is still employed in the conversion. In some designs of conversion sears therelease lever is integrated into the sear design, so eliminating one extra part. And whether or not the conversionused a modified originally semi-auto onlypack frame or an adapted, cut and blocked, originally select-fire packframe, the conversion geometries of the newly installed mechanics are the same!

Conversion SearsIts all in the T iming!

As we have discussed above, the lower frontal area used to clip-on the front mounting ledge on a semi-automatic fire control pack is where the factory original select-fire, fire control pack mounts the pivot pin holesfor the automatic release sear. Since the auto sear pivot pin is also where the trigger return spring is mounted,HK GmbH in its redesign to semi-automatic-only operation selected a new location for the trigger return spring.It was now riding on a pivot pin used only to mount the return spring and its redesigned semi-auto onlydistance sleeve. This new return spring location in the semi-auto only pack is the location that conversion searsare mounted in, the same effectual location as in a factory original select fire pack. NFA Conversion fire controlpacks, either original semi-auto or modified select-fire versions, employing either a registered or unregisteredconversion sear, utilize that relocated trigger return spring pivot pin hole location. That relocated pivot pinposition for the new conversion sear has a veryimportant effect on mechanical functioning.

On original factory select-fire packs the auto sear pivot pin holes are located 7mm farther forwardin relation tothe trigger pin pivot hole, and 3mm lowerthan in conversion packs. This rearward relocation of the searengagement to hammer contact surface in conversion packs has the net effect of causing the hammer to be held

in a different vertical position than on a factory select-fire pack. With the hammer cocked and held by the twoversions of auto sear (catch), there is a large difference in allowable hammer travel upon auto sear release. Ona converted pack the hammer rides 12mm higher than in a factory select-fire original pack. This difference inhammer travel to firing pin contact is equivalent to lock time; a decrease in lock time will cause an increase incyclic rate!This is what is commonly known as the timing of the conversion, and proper timing is critical tomore than just mimicking factory cyclic rates. Remember, the original factory design uses the auto sear also asan effectual safety sear, preventing inadvertent hammer release until final bolt battery lock up. Since the gunemploys a delayedblow back action using roller locking, the onlything preventing a totally unlocked cartridgeignition is if the bolt rollers have entered their locking recesses. This is theoretically only possible if the lockingpiece can travel to full extension by forcing the rollers outward allowing the firing pin tip to protrude out of thebolt face. However, in certain situations the locking piece can forced under hammer spring pressure to contactthe firing pin before completeextension of the bolt rollers, particularly in a gun with worn components such asthe locking recesses in the barrel extension. As such it is critical that the auto sear release occurs only aftercomplete extensionof the bolt locking rollers into their locking recesses. This timing is particularly effected by

two things: firstly, the release lever camming trip surface on the underside of the bolt carrier, secondly by thehammer-sear contact geometry. The release lever camming timing is merely a matter of restoring the originalfactory bolt carrier underside contours, something that was usuallydone by most manufacturers of theseconversions. Restoring the hammer-sear contact interface geometry is considerably more difficult to achieve.Due to the relocation of the new conversion auto sear, if an unalteredfactory original hammer is employed thenew conversion sear will hold the hammer in the higher position as described above. This is an inducedreduction in lock time, and if combined with an improperly re-contoured bolt carrier tripping surface, it maycause early release of the hammer as opposed to factory specifications. If this occurs in a well worn gun withworn locking recesses the bolt rollers may notbe fully extended when the hammer spring forces the lockingpiece forward to contact the firing pin. For this reason a properly converted hammer used in a sear conversionwill have had the factory original sear notches cut on the front lower face of the hammers welded up and the re-cut to allow for the new geometry of the relocated conversion sear. Unfortunately, not all sear conversionshave had this accomplished. While the safety reasons as explained above should be enough, the most common

reason desired to have the hammer re-contoured is the restoration of the factory specified cyclic rate for theparticular gun model in question. Even a minor increase in cyclic rate will cause a large increase in recoil forcesand induced wear and tear, something to consider since these guns are often extremely expensive now.

(Note: there have been observed slight variations in these actual geometries depending upon the individualconversion in question. These measurements are representative of industry accepted practices for alterationsrequired to adapt these conversions, individual guns may exhibit minor variations that should notbe considered




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a defect if the gun runs acceptably. An insignificant variation in cyclic rate is the usual net effect)

There are a few more minor modifications required to get these conversion sears to properly function. Theoriginal disconnector (sear in HK parlance) must be slightly relieved to allow it to properly function in the newmechanical arrangement. A sear conversion converted pack uses a factory original machine gun distancesleeve, as the conversion sears occupy essentially the same dimensions as the factory original catch, and so thelonger semi-auto only distance sleeve cannot be used. The semi-auto only and factory select fire trigger (andauto sear) return springs are also different. The auto sear (or catch) release lever is almost always used in itsfactory original form, and location, the design of the NFA conversion sear accommodating this arrangement.

Conversion Sears: Equivalent, but often different

We have been discussing those NFA conversions that retain an unmodified Title I style receiver and attain selectfire operation by utilizing a conversion part known almost universally as a conversion sear in nearly genericterms. There were only a relative few proprietary designs of these conversion sears, yet there are someimportant distinctions to be made.

As discussed, NFA conversion sears may be divided into two main categories, those that exist as a directreplacementfor the factory designed original catch and are designed to function in the exact same manner if ina slightly different geometry by using a factory type release lever. And secondly, there are those styles that aredesigned to replicatethe factory mechanics by combining the functions of the factory catch and the release leverin one conversion part. Again, depending upon the individual Class II Manufacturers or Qualified Individualspersonal choice at the time of conversion, or the timing of the registration involved, these conversion parts mayor may notbe individually registered in the NFRTR as machine guns in and of themselves!! They do howeverall function identically and may exist in exactly the same physical form with some being NFA registeredthemselves, and some being installed onto gun receivers that are the registered item as described above.

The first kind of conversion sear is designed to directly replace the factory catch, or auto sear. Despite themethod or materials of manufacture they are identical in all significant mechanical aspects. The factory catcheswere initially designed to function with a curved end finger of the upper arm extension acting against thestraight undersurface of the release lever by riding in a cammed arc in direct friction contact. As such theybecame known as friction catches. This, as opposed to an updated version which employed a catch that wasdesigned with a small roller wheel at the extreme end of the extension arm, acting against a curved underside ofa redesigned release lever arm, these are known as roller catches. All NFA conversion sears of the directreplacement type were made in the friction catch style!!

Direct replacement type conversion sears were made of steel and by casting, stamping, or machining of barstock depending upon individual choice or manufacturing ability at the time. Quality and durability vary, andthough all are serviceable there are differences. Several physical styles of these direct replacement sears werecreated. Some are dimensioned and shaped very differently than the factory friction catch they are designed toreplace, and as such they may require proprietary changes to the other trigger group parts that are slightlydifferent than what would be considered industry standard as described above.

The most desirable types are constantly being argued, but if replication of factory design, dimensioning, andmaterials are considered there is one clear winner: the stamped and formed conversion sears are hands downclosest to a factory friction catch, and are regarded as such. Despite construction type and materials the singlemost important factor in durability of any of these conversion sears is proper heat treatment. It is known thatsomeof the machined or cast sears were not heat treated correctly, if at all, and they will exhibit excessive wearprofiles. Most of these are known to be installed as unregisteredconversion sears residing in conversion packsmounted to receivers that are themselves NFA registered. Fortunately, any of these incorrectly heat treatedsears can be re-heat treated by a competent and knowledgeable Gunsmith. Additionally, the upper armextension finger can exhibit excessive wear, galling, or cracking. The only other problem to watch for is that

some of the cast sears have been found that have improperly located pivot pin holes, and these off-axis pinholes can be worn out of round over time. This too can be fixed rather easily. Those stamped and formedconversion sears produced in a tandem manufacturing effort by S&H Arms and Fleming Firearms, and the castsears by S&H Arms are regarded as the most likely to offer effectual service and lasting value.

When moving into the realm of the combined catch with release lever type of conversion sear we find a muchbroader interpretation of what constitutes a conversion part as mechanically and legally defined. These aregenerally speakingfound to be of the earliest era of NFA conversions and exist in physical forms and dimensionsthat are generallytotally different than a factory part. As described earlier, many Class II Manufacturers wereforced to modifythe existing semi-auto-only parts that came in the to-be-converted gun because of the total lackof available factory select fire parts then available on the open market. Accordingly many of these convertedsemi-auto parts resemble Modern Art masterpieces!! At least one competent Class II manufacturer, Jonathan

Arthur Ceiner, Inc., essentially replicated the factory combined catch and release lever as found employed in theHK21E/21A1 LMG into a proprietary design that is exceptionally well done. It is generally found though, that

most of these combined function conversion parts are far from an ideal set-up. It is impossible to offer muchserious advice separate from the general advice given above, as there are just too many varying styles installedby manufacturers with widely varying mechanical competencies. Except as stated above for the singularexception for Ceiner conversion sears, these combined function sears are regarded as being the lowest value interms of desirability.

Through the Glass, Darkly.




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With these NFA conversions employing a gun receiver with the Title I style clip-on style trigger groupattachment point there remains a very tempting trap awaiting those whom would knowingly or unknowingly slipinto it. Why this is may not be readily apparent, and even then there remains some stubbornness in acceptingthe legalities of the current situation regarding this. The desire to update their gun with a newer style plastic

Ambidextrous or 2-shot/3-shot Burst style trigger group remains almost universally strong among currentowners, however there needs to be issued one final warning regarding doing this.

The ONLY LEGAL WAYto install a newer style Ambidextrous or 2-shot/3-shot Burst style trigger group is ift h e original, registered or unregistered, conversion sear (or catch) can be mated to function in the

Ambidextrous or Burst trigger pack mechanics. This means you must discard the factory sear (catch) that

came in the new pack and affect installation of the original registered or unregisteredconversion sear that wasutilized in the original fire-control pack. Some few proprietary sear designs can be adapted to fit and made toproperly function, most cant.

As detailed earlier, the conversion pack frame itself has to have been altered to fi t the clip-on semi-auto receiver,while having the new style plastic housing adapted to fit the clip-on receiver, and then blocked to prevent re-installation of an original select-fire trigger pack. Now often enough this is done with the newer style plastic S-E-F housings which are essentially identical in function to the older style metal S-E-F versions, and this poses nolegal problems by doing so. Because mostof the direct replacement style conversion sears are modeled afterthe friction sear found in the factory original metal or plastic housing style S-E-F trigger groups, adapting justthe plastic housingpresents no mechanical problems.

The legal and mechanical problems arise when we are trying to adapt a newer style Ambidextrous or 2-shot/3-shot Burst style trigger group.And there is a very good reason not to simply go ahead and have such an

adaptation performed by simply discarding the originally installed conversion pack and clipping on a newlyadapted one. The mechanics of the Ambidextrous or burst trigger groups are VERYdifferent than the standardS-E-F type of fire control pack, with many of the component parts totally redesigned to properly function withthe burst counter clockwork mechanism installed. Ambidextrous style trigger groups only offering selective, butnot burst, fire modes are also arranged identically and so suffer this same problem. H&K GmbH actually offeredtwo very distinct styles of burst mechanisms. The first version externally resembled the S-E-F type of housingand located the burst clockwork mechanism in the rear lowerportion of the pack frame. This is significantas itleft the front catch position identical to the S-E-F fire control pack!! This means that this style burst pack, andonly this style, can be readily adapted to function with those conversion sears resembling externally the factoryfriction catch!! These very rarefire control packs command an extreme premium in todays market as they canbe easily adapted to function with the most common versions of conversion sears, and the market for usingthem to install registeredconversion sears primarily, is very strong. The second style of burst pack located theclockwork mechanism more toward the frontal region of the pack frame and this forced a complete redesign of

the internal components. These new style burst packs cannot be easily or readily converted to use a NFAconversion sear!! Many styles of proprietary NFA conversion sears simply cannotbe adapted to function withoutmajor mechanical redesign. Due to this fact, there is a large temptation to avert the hassles in doing so bysimply adapting the entire trigger group and exchanging it on a Registered Receiver guns receiver thatformerly used an unregisteredconversion sear. Attempting to do so leaves one extremely vulnerable in a legalsense.

One must remember that when doing semi-auto conversions was a lawful enterprise, there were not that manyshops actually performing these jobs, and the vast majority of guns converted were done by a relative few. Itbecame quickly known which shops preferred to do which manner of conversions, and by what methods theycommonly converted these guns, so a savvy ATF Field Agent can easily tell if a particular gun should have comewith a particular type trigger housing, or fire control pack, etc. Also, the easy availability of the new style

Ambidextrous/Burst trigger groups today was not the caseeven a few short years ago, so the likelihood of one

having been originally installed is not too great. BE FOREWARNED!! And be fully prepared to PROVE that yourNavy Group, or Burst trigger group now residing on your clip-on style Registered Receiver conversion guncontains, and has been adapted to function with, the original unregistered conversion partthat the gun camewith...and yes, it HAS happened.

Losing the original fire control pack, and its original unregistered conversion part, for a newerAmbidextrous/Burst style trigger group is a good way to lose the whole expensive gun...and possibly more.

So...Now you know!

Hopefully now, there is a clearer understanding of just how complex the topic of NFA Conversions on H&K styleweapons is! It is a fact that since these guns represent one of most modern and desirable types of Civilian legalmachine guns available, many enterprising Class II manufacturers sought to do conversions on these guns.Unfortunately, all of these attempts were not created equally. Therefore, it is paramount to assess the basic

conversion type in question before a fair market value can be assigned in this quickly escalating market place ofrecent years.

(All content Copyright 1999 by Thomas T. Hoel)




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HK NFA Conversions - Machine Gun Price Guide

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