(2) Firearms and Toolmarks
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Transcript of (2) Firearms and Toolmarks
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Firearms and Toolmarks
Types of Firearms
Shotguns
Rifles
Handguns
12 gaugeshotguncartridge
rifle cartridge7.62 mm
handguncartridge44 MAG
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Cartridge Components
Projectile(bullet)
cartridge case
gunpowder(nitrocellulose)
igniter
Four Major Classes of Bullets
Plain lead
Partially jacketed lead
Fully jacketed lead
non lead (bronze, plastics, wax)
Shotgun Cartridge Components
cartridge
powdercharge
00 buckshotprojectiles
foam buffer
base wad
filler card
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Firing Sequence 1• Firing pin strikes primer, primer ignites
Firing Sequence 2• Powder is ignited by primer flame
Firing Sequence 3• Burning powder forms gases, as gases expand, projectile is launched
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6 Land-and-Groove Barrel
A firearm barrel is a tool that leaves striated toolmarks on a fired bullet
Internal Ballistics
• Grooves in the barrel of the firearm called rifling cause the bullet to spin as it moves through the barrel
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External Ballistics
• Barrel spin reduces drag and increases the stability of the projectile through air
• Projectiles without barrel spin will tumble end over end
What can you determine from a fired bullet?
1. Direction of twist
2. Number of grooves
3. Caliber or gauge
4. Width of lands and grooves of barrel
5. Possibly gun manufacturer(s)
Class Characteristics
General rifling characteristics (GRC) - The number, width, and direction of twist of the rifling grooves in a barrel of a given caliber firearm.
A firearm barrel also leaves land and groove impressions on bullets
cannelure
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Recovery of Test fired Specimens.
Test firing a suspect firearm in the water recovery tank
Water recovery tank
Comparison Microscope
Identification using a breech face
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Striated marks left on fired bullet
(Individual characteristics = identification)
Microscopic marks inside a gun barrel
LAND
GROOVE
Bullets fired from different barrels
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Bullets fired from same barrel
Comparison Microscope
Close-up / Land Impression
40X
Land Impressions
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Comparison Microscope
•The most valuable equipment used by the examiner in order to perform comparative analysis
•Two stages in the same view
Comparison Microscope
Firing Pin Impression
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Comparison Microscope
•The evidence casing located at the scene placed on the left
•The test fired casing from suspect firearm placed on the right
10X
Comparison Microscope
By placing the casings next to each other
Side by side the similarities are observed
15X
Comparison Microscope
shearing
20X
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Comparison Microscope
40X
Comparison Microscope
FBI Firearms Collection
5,700 Specimens in the firearms collection
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Ammunition collection
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Gunshot ResidueVisual inspection
Microscopic inspection
Chemical testing
• Determine presence or absence of primer, gunpowder or lead residue on victim’s clothing
• Determine approximate distance from firearm muzzle to victim’s clothing
PISTOL CARTRIDGE
RIFLE CARTRIDGE
RIMFIRE CARTRIDGE
Variation in bullet and powder types are important variables in residue deposition
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Gunshot Residue Analysis on Victim Clothing
Chemical Processing for Gunshot Residues
Test fired pattern
The Modified Griess Test is a test to detect the presence of nitriteresidues.
Sodium Rhodizonate Test is designed to determine if lead residues are present on the exhibit.
Test fired reproduction from a suspect’s firearm
When a cartridge is discharged in a firearm, unburned or partially burned gunpowder, as well as other residues associated with a gunshot escape from the firearm. These residues can be deposited on garments at reasonably close ranges.
Reaction on test material fired at 9 inches Greater distance = Greater spread
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Chemical test result from contact shot:Notice heavy concentration of vaporous leadGunpowder blasted through hole (wound)
Primer Residue Analysis on Suspect’s Hands
Barium, antimony and lead by SEM and X-ray analyzer
Cartridge primer contains barium nitrate, antimony trisulfide and lead styphnate
Sample collection within four hours
Primer Residue
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Obliterated Serial Numbers
Acid Etching Restoration
Magnetic Number Restoration
Large magnet
Magnaflux
Remington shotgun receiver with serial number obliterated
Number restored using Magnaflux
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Thermal Restoration
Used on cast iron engine blocks and some firearms
Apply heat in a circular motion
Restoration of obliterated numbers
SA7 painted to obliterate identifying numbers
IR photograph of the same SA7 missile tube
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Class characteristics – spacing of the teeth
Marks created by same drill bit
Marks produced by consecutively made drill bits
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Bullet path reconstruction
Bullet Trajectory Reconstruction
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Trajectory analysis