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Firearms, Took Marks &

Other Impressions

Chapter 15

Firearms: A Quick History

• Almost every gun is based on the same

simple concept: You apply explosive

pressure behind a projectile to launch it

down a barrel.

• The earliest & simplest application of this

idea is the cannon.

The 1st handheld guns were essentially

mini-cannons; you loaded some

gunpowder & a steel ball & lit a fuse.

• War typically resulted in the need for

improved weapons technology.

• In the late 1800’s, the revolver quickly

became popular due to it’s size & quick

loading.

– It only had to be reloaded every 5-6 shots

instead of after each shot.

• Handguns reigned supreme for the past

200 years & to this day, remain the most

popular & readily available firearm.

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Firearms Identification

• Often confused with the term ballistics

• Ballistics is the study of a projectile

in motion.

– Inside the firearm

– After it leaves the firearm

– When it impacts the target

Firearms Identification

Identification of Firearms is based

upon this basic idea:

A harder object marks a softer one &

imparts/transfers its microscopic

irregularities to that object.

Forensic Firearms Expert

• Did a suspect use this gun to kill that

person?

• Did these bullets come from that gun?

• Was it really self-defense?

• Is this a case of suicide, or is foul play

involved?

Forensic Firearms Expert

• Bullet Comparison

• Weapons Function

– Is it safe? Has it been modified?

• Serial Number Restoration

• Gunpowder Residue Detection

– on clothes, hands, & wounds

• Muzzle-to-Target Distances

Pulling the Trigger

• Pulling the trigger releases the firing

pin…

• The firing pin strikes the primer…

• The primer ignites the gun powder…

• The powder generates gas that propels

the bullet forward through the barrel &

ejects the spent cartridge case.

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Caliber: the diameter of the gun barrel.

• Caliber is recorded in

– hundredths of an inch (.22 & .38)

– millimeters (9mm)

Bullet Anatomy Anatomy of a Bullet

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Bullet Comparisons

• Each gun leaves distinct markings on a

bullet passing through it.

• A gun barrel is made from a solid bar of

steel that has been drilled/hallowed out.

• The drill leaves microscopic marks on the

barrel’s inner surface.

Bullet Comparisons

• Gun manufacturers also add spiral

grooves to the barrel. This is known as

rifling.

• Lands: the space between the grooves.

• As a spinning bullet passes through the

barrel, it is marked by these grooves.

Class Characteristics

• Class Characteristics: Once a manufacturer

chooses a rifling process, for a particular class of

weapon, they keep it consistent.

• Lands & Grooves are the same for a model.

– .32 caliber Smith & Wesson has 5 lands & grooves

twisting to the right.

– .32 caliber Colt has 6 lands & grooves twisting to the

left.

• Class characteristics can eliminate certain

makes but are not enough to ID a particular gun.

Individual Characteristics

• Imperfections in the manufacturing

process make each barrel unique.

• Rifled barrels, even if made in succession

will NOT have identical striation (scratch-

like marks).

Bullet Comparisons

• To match bullets to a gun, test

bullets must be fired through a

suspect barrel for comparison.

• Goddard & Comparison

Microscopes

– Examined bullets side-by-side (to

match striated markings).

Bullet Comparisons

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Cartridge Markings

All moving components contact the

cartridge rather than the bullet can

leave useful impressions on shell

cartridges

– Breechface marks

– Firing pin impressions

– Chamber marks

– Extractor & Ejector markings

Breechface Marks

Actual Breech Marks Firing Pin Marks

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Chamber MarksEjector Marks

Other Factors• Perfect matches sometimes difficult b/c:

– Presence of grit & rust in a barrel

– Recovered bullets too mutilated or distorted on impact

• A spent bullet’s weight can sometimes determine the gun make.

• Microgrooves: 8-24 grooves; it’s not as common

• General Rifling Characteristics File

– FBI database of known land/groove width for all weapons.

Shotguns

• Smooth barrel

– Projectile NOT marked as it passes

through

• Fire small lead balls or pellets

contained within a shell.

• Characterized by:

– diameter of the shot

– size & shape of the wad

– Gauge: diameter of the barrel

• ( gauge diameter)

Gunshot Residue (GSR)

• GSR Sources:

– victim, clothing or target

– shooter’s hands

• Gunpowder Chemistry

– Major detectable elements are: lead

(Pb), barium (Ba) & antimony (Sb)

– Virtually all cartridge cases are made of

brass (copper & zinc); also detectable.

Griess Test

• Tests for the presence of nitrates (partially

burned or unburned gunpowder)

• Swab of shooter’s hand

• Must produce a pattern for a distance

determination

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Results of GSR Hand Test

• Negative results may be caused by:

– Washing the hands

– Shooter may have been wearing gloves

– Lead free ammunition

• A rifle or shotgun may not deposit GSR on

hands

GSR on the hand of a suicide victim, proving

he was holding the weapon when it was fired.

Trajectory

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SERIAL NUMBER RESTORATION

– When a serial number is stamped into a gun,

the metal underneath the number is

compressed & hardened.

– If the number is filed-off, the hardened area

may still be present.

– By using an acid solution the metal can be

slowly eaten away.

• In this process the softer metal will be eaten away

first and the number may reappear.

Before & After

– Historically, serial numbers are successfully

restored 63% of the time on steel & 54% of

the time for aluminum or zinc.

FIREARMS EVIDENCE

COLLECTION

• Make sure it is unloaded!!!!!

• DO NOT put a pencil into a barrel

• REVOLVERS

– Indicate location of fired & unfired ammunition

• AUTOMATICS

– Check magazine for number of rounds

– Fingerprint magazine

• Place ID tag on trigger guard

FIREARMS EVIDENCE

COLLECTION

• AMMUNITION

– Write on base or nose

– Package in pill box or envelope

– Wrap in tissue to protect

FIREARMS EVIDENCE

COLLECTION

• CLOTHING

– Protect & preserve any residue

– Air dry if wet

– Package separately in paper bags

• Establish CHAIN OF CUSTODY

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TOOL MARK IMPRESSIONS

•Impressions

•Cuts

•Gouges

•Abrasions

TOOL MARK IMPRESSIONS

•SUBMIT ENTIRE OBJECT

•PACKAGE SEPARATELY (ALWAYS!)

•DO NOT ATTEMPT TO FIT THE TOOL

INTO THE TOOL MARK

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