Gauging Headspace onAK Pattern Rifles

VSO-00V2

R. Forbus, C. Hallstrom, J. Houlton,B. Keeney

Contents

1 Executive Summary 2

2 Scope 2

3 Introduction 2

4 Equipment 3

5 GO Gauge Testing 45.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

6 NO-GO Gauge Testing 56.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7 FIELD Gauge Testing (For End Users) 67.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

8 Acknowledgements 7

9 Disclaimer 8

10 Endorsements 8

11 Appendix 811.1 Parts Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

©2019 VSOrdnance LLC. - www.VSOMedia.org VSO-00V2-1

Gauging Headspace onAK Pattern Rifles VSO-00V2 R. Forbus, C. Hallstrom, J. Houlton, B. Keeney

1 Executive Summary

This procedure is a comprehensive guide for checking the headspace on AK pattern rifles. Theintended audience is the end-user, someone who is not necessarily technical but is capable offollowing directions and knows how to field strip an AK and dissassemble the bolt.

2 Scope

This procedure applies to AK pattern riles built on both stamped and milled receivers. Please notethat rifles built using single-piece, milled receivers do not have trunnions. References to thesecomponents in this procedure should be interpreted to mean the analogous portions of the receiver.

3 Introduction

This document describes a procedure widely accepted by US professionals for checking theheadspace of AK pattern rifles. Headspace is the length between the bolt-face and the swell of thecartridge. If headspace is too short the bolt cannot completely close, leading to out-of-battery firing,which is very dangerous. If headspace is too long, the cartridge is not adequately supported, whichcan lead to burst casings. Over-length headspaces can also lead to the bolt running rearwardsinto the locking lugs on the trunnion in a way the design was not intended to support. This canlead to progressive wear patterns that can ultimately result in the shearing of lugs from the bolt ortrunnion, causing severe injury to the operator.

Figure 1: Example of Bolt Lugs and Cartridge.

The headspace of a rifle changes over time. It is initially set at the time of build by pressing thebarrel into the trunnion and potentially tuning (with a file or stone) the locking lugs on boththe bolt and trunnion. The match between the trunnion and bolt (determined by layout dyeapplication of the faces) should be 80% (must be greater than 60% [1]), but it is not uncommonto observe AKs with less than this overlap. In guns with rougher machining (and lower percentagematching between lug and bolt face) on trunnions and bolts, the headspace will grow rapidlywith gun break-in, tapering slowly upward as time goes on. We recommend lubricating the backfaces of the bolt to minimize this wear. It should be stressed that the headspace will grow in allrifles over time. Assuming identical heat treat and metallurgy, well fitted bolts and trunnions will

VSO-00V2-2 ©2019 VSOrdnance LLC. - www.VSOMedia.org

VSO-00V2 R. Forbus, C. Hallstrom, J. Houlton, B. KeeneyGauging Headspace on

AK Pattern Rifles

change less over the lifetime of the rifle than those that start rough. Replacement componentsshould be installed and fitted by a gunsmith competent in AK practices.

Figure 2: Fitment of Lugs

There are three gauges available for testing the headspace: GO, NOGO, and FIELD. The GO andNOGO gauges are only appropriate for use in the factory and up to the point that the rifle is firedfor the first time. Assuming that the gun was built properly, the GO gauge is not needed at all afterfirst firing. The NO-GO gauge can be used to track the wear of a rifle, but it’s not an indicationof whether the gun is safe or not. The true indicator of safety (assuming the gun passed GO andNOGO at the time of manufacture) is the FIELD gauge. If a rifle closes on a field gauge it hasreached the end of its service life and should be rebuilt or rendered inoperable to prevent injury tounwitting users.

Figure 3: Premature Wear Resulting in Growing Headspace

4 Equipment

� CIP GO Gauge of appropriate caliber (5.45, 5.56, 7.62) (See Appendix for sources)� CIP NO-GO Gauge (5.45, 5.56, 7.62)� CIP FIELD Gauge (5.45, 5.56, 7.62)

©2019 VSOrdnance LLC. - www.VSOMedia.org VSO-00V2-3

Gauging Headspace onAK Pattern Rifles VSO-00V2 R. Forbus, C. Hallstrom, J. Houlton, B. Keeney

� Force Gauge capable of reading up to 50 kg (NK-500 for example)� Optional known mass between 5 and 25 kg (10-50 lbs) for rough calibration of scale� Chamber brush of appropriate size� CLP for cleaning the chamber� 7" dowel or tubing (pipe) or stopper that will fit in recoil spring hole on bolt carrier, protruding

.5-1 inch.� Blow-off gun or lint-free wipes for cleaning the chamber.� Magazine Bench Block in vise or other device for securing the rifle in a horizontal position.

5 GO Gauge Testing

5.1 Introduction

This portion of the procedure covers checking the headspace on a new rifle with the GO Gauge. TheHeadspace must be large enough to admit all cartridges and close reliably on them. Headspaceswhich are too tight may cause out-of-battery firing, where the bolt is not adequately locked. This isa hazardous condition, and should be avoided at all costs. That said, one does not want a loose fiton a no-go gauge at the time of manufacture. While it is safe, it implies that there is less availablemargin for the bolt lug faces and trunnion faces to wear before allowing a close on a field gauge,which indicates end-of-life.

5.2 Procedure

� Make sure the gun is unloaded, and all ammunition has been removed from the area. Checkvisually and feel for an EMPTY CHAMBER.

� Remove the dust cover, recoil spring, and bolt carrier. Remove the bolt from the bolt carrier.� Dissassemble the bolt, removing all pins, extractor claw, and springs. Store these in a bag or

container for safe keeping.� Clean the chamber with a small amount of CLP and the bottle brush. Ensure that the bolt

face is likewise clean.� Blow or wipe both the chamber and the bolt face. Wipe down the GO Gauge, ensuring no

grit or debris are present.� Insert the GO gauge into the chamber.� insert the bolt and bolt carrier into the rifle, ensuring that it will glide freely after going over

the hammer in its forward travel up to contact with the GO Gauge.� using a stopper placed in the recoil spring hole, press forward on the bolt carrier using the

force gauge and in one smooth motion press forward with up to 15 kg of force registered onthe gauge. See Figure 5

� The bolt carrier must touch the trunnion. If possible record the peak required force.� Alternate practice: Insert the bolt behind the GO Gauge. Rotate the bolt clockwise with firm

pressure using fingers (no tools). For new production, the ideal is a tight close on a GOGauge. Some manufacturers will leave a 2.5-3 mm gap between the triangular lug and thefire control side of the front trunnion.

VSO-00V2-4 ©2019 VSOrdnance LLC. - www.VSOMedia.org

VSO-00V2 R. Forbus, C. Hallstrom, J. Houlton, B. KeeneyGauging Headspace on

AK Pattern Rifles

Figure 4: Triangle Lug Spacing

6 NO-GO Gauge Testing

6.1 Introduction

In this portion of the procedure we attempt to find a middle-ground between the various testingtechniques. We hope that most, if not all, parties find it reasonable, even if it’s not to theirparticular taste. We owe this portion of the technique to Robert Forbus, who derived it from anEast German armorer’s manual [1].

6.2 Procedure

� Make sure the gun is unloaded and all ammunition has been removed from the area. Checkvisually and feel for an EMPTY CHAMBER.

� remove the dust cover, recoil spring, and bolt carrier. Remove the bolt from the bolt carrier.� Dissassemble the bolt, removing all pins and springs. Store these in a bag or container for

safe keeping.� Clean the chamber with a small amount of CLP and the bottle brush. Ensure that the bolt

face is likewise clean.� Blow or wipe both the chamber and the bolt face. Wipe down the GO NOGO gauge, ensuring

no grit or debris are present.� Insert the NO-GO gauge into the chamber.� reinsert the bolt into the bolt carrier and install the bolt carrier in the rifle, leaving the recoil

spring off.� Move the bolt carrier close to the rear of the rails, but not so far that it can pop out.� Power up the force gauge and verify calibration with a known mass.� Place a plug or dowel in the recoil spring hole in the bolt carrier.� In one smooth motion, press the bolt carrier via the stopper towards the muzzle such that

when you reach the end of travel the scale has reached a maximum of 20 kg. See Figure 5.� Ideally, the bolt doesn’t begin to close. If it partially closes that’s still acceptable, but if it

closes all the way it is a failing test. If the bolt rotates do your best to note the angle ofrotation from the rear.

©2019 VSOrdnance LLC. - www.VSOMedia.org VSO-00V2-5

Gauging Headspace onAK Pattern Rifles VSO-00V2 R. Forbus, C. Hallstrom, J. Houlton, B. Keeney

6.3 ConclusionThis test is only really valid BEFORE the gun has been broken in. Several factors contributeto headspace growth. One of them is roughness and co-planarity (or lack thereof) betweenthe locking lugs on the bolt and trunnion. If they are out of alignment or rough, they willwear faster (and thus grow headpspace faster) than polished lugs with greater coplanarity.Another legitimate use of the NO-GO gauge test is for judging the wear of the lugs; however,this is not a judgment of safety. The two absolute measures of safety are the GO and FIELDGauges.There is another reasonable interpretation of the NO-GO fail that should be discussed.Imagine that the gun closes on a NO-GO gauge, but does not close on a field gauge. It’s stilltechnically safe to shoot. However, at some point the gun will close on a field gauge. Thisby definition means that the gun has been fired in an unsafe condition. So if the reader islooking to ensure that they never shoot an unsafe gun, even at the expense of early retirement(and thus cost), the logical choice is to retire the gun upon the NO-GO closure.Yet another method is to periodically use the alternate (bolt carrier free) method, whereone measures the distance between the camming lug and the side of the trunnion with thethree gauges progressively as the gun ages (see figure 4). This distance, tracked over time, isprobably the most fine-grained and accurate approach to assessing the aging process of thegun.

7 FIELD Gauge Testing (For End Users)

7.1 Introduction

The FIELD gauge checks for the absolute largest allowable headspace. If the headspace gets toobig, the cartridge case can separate and the trunnion or bolt can fracture, leading to injury ordeath.

7.2 Procedure

� Make sure the gun is unloaded and all ammunition has been removed from the area. Checkvisually and feel for an EMPTY CHAMBER.

� Remove the dust cover, recoil spring, and bolt carrier. Remove the bolt from the bolt carrier.� Dissassemble the bolt, removing all pins and springs. Store these in a bag or container for

safe keeping.� Clean the chamber with a small amount of CLP and the bottle brush. Ensure that the bolt

face is likewise clean.� Blow or wipe both the chamber and the bolt face. Wipe down the FIELD gauge, ensuring no

grit or debris are present.� Insert the FIELD gauge into the chamber.� Reinsert the bolt into the bolt carrier and install the bolt carrier in the rifle, leaving the recoil

spring off.� Move the bolt carrier close to the rear of the rails, but not so far that it can pop out.� Zero the force gauge or hanging scale and verify calibration.

VSO-00V2-6 ©2019 VSOrdnance LLC. - www.VSOMedia.org

VSO-00V2 R. Forbus, C. Hallstrom, J. Houlton, B. KeeneyGauging Headspace on

AK Pattern Rifles

� Place the plug into the recoil spring bore and insert the pointed tip of the force gauge(alternatatively the hook of the scale in the hole where the recoil spring goes), and orient thescale along the axis of the bore (You did clear the gun and remove the firing pin, right?). Seefigure 5 for an example of the force gauge setup.

� In one smooth motion (DO NOT get a "running start"– this is NOT an impact test!), push/pullthe bolt carrier towards the muzzle using the hook of the scale, such that when you reachthe end of travel the scale is reading between 18 and 22 kilograms (nominal 20 kilograms).

� Ideally, the bolt doesn’t begin to close. If it partially closes that’s still acceptable, but if itcloses all the way it is a failing test and should be decommissioned.NOTE:It should be noted that it is possible to deform the gauge, the chamber, or both by performingthis procedure improperly. Evaluation should occur with target components and the fieldgauge at relatively the same ambient temperature to ensure thermal expansion does notyield a dangerous false positive result.

Figure 5: Field Gauge Under Tension

8 Acknowledgements

The authors owe a great debt to the wisdom and teaching of Jim Fuller, Robert Forbus, Marc Krebs,and John Houlton. Thank you gentlemen, for sharing your time and knowledge with us.

©2019 VSOrdnance LLC. - www.VSOMedia.org VSO-00V2-7

Gauging Headspace onAK Pattern Rifles VSO-00V2 R. Forbus, C. Hallstrom, J. Houlton, B. Keeney

9 Disclaimer

This procedure is for information purposes only. Occam Defense Solutions Inc., VSOrdnance LLC.,and all other entities appearing below expressly reject all liability and responsibility for any injuriesor damages arising out of proper or improper use of this procedure.

10 Endorsements

We, the undersigned, endorse this procedure as one (not the only) legitimate way to test headspaceon AK Pattern Rifles:

Robert Forbus, Ing. Robert Forbus Date

Curtis Hallstrom, VSOrdnance Date

John Houlton, M13 Industries Inc. Date

Brian Keeney, Occam Defense Date

11 Appendix

11.1 Parts Sources

• A suitable and relatively inexpensive force gauge is the NK-500 Mechanical Analog Push PullGauge Force Gauge available from Amazon.com

• Suggested Source for AK Headspace Gauges: AK-Builder.com• Bore Brushes and CLP can be found at Brownell’s and many other sources.

References

[1] English Translation by John Baum. AK-47 Repair; A 050/1/506 7.62mm Machine Pistol KM,KMS-72, and KmS Repair National People’s Army of the German Democratic Republic, 1978.

VSO-00V2-8 ©2019 VSOrdnance LLC. - www.VSOMedia.org

J. Robert Forbus 04, April 2019

4/4/2019

2019-04-06