(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