Ch.9 Firearms, Tool marks, and other Impressions

three class characteristics of a gun barrel

Caliber, number of lands and grooves, and the direction/twist of the lands and grooves.

Describe how a firearms examiner compares two bullets and what characteristic is most often used and why

They compare bullets under a comparison microscope to find matching striations; striations are used because no two rifled barrels produce identical markings.

List two reasons why striations on bullets from the same gun may vary slightly

Grit or rust can alter markings, and striations naturally change as the barrel wears.

Besides the barrel, what firearm parts may leave distinctive markings on a cartridge

Firing pin, breechface, extractor, ejector, magazine, clip, and chamber walls.

Why a firearms examiner test-fires bullets from a suspect barrel

It's the only practical way to directly compare barrel markings to those on a recovered bullet.

What is distance determination and two situations where it matters

Determining how far the gun was from the target; used in evaluating self-defense claims and distinguishing suicide from homicide.

What evidence is used in distance determination and how test-firing helps

Powder-residue patterns around the bullet hole;

test-firing produces known patterns to compare and estimate distance.

Three characteristics of a close-range bullet hole

Heavy vaporous lead, scorched or melted fibers, and stellate (star-shaped) tearing.

What is the Greiss test and what two things it shows

A chemical test showing presence of gunpowder residues and helping estimate shooter-to-target distance.

How shot pattern is used for shotgun distance determination and what affects it

Wider pellet spread means greater distance; affected by barrel length, pellet size/quantity, powder charge, and choke.

What evidence indicates whether a suspect fired a handgun and where it's found

Primer residues found on the thumb web and back of the firing hand.

Why primer-residue analysis gives low positives and is ineffective for .22 caliber

Residue is easily removed and dissipates quickly; .22 rimfire primers often lack detectable primer elements.

How an obliterated serial number is restored

An etching agent dissolves altered metal faster than intact metal, revealing the original number.

Why a weapon should not be picked up by the barrel and how it should be handled

It can disturb deposits and alter evidence; handle by the edge of the trigger guard or checkered grip.

What firearm characteristics should be recorded before unloading and why number chambers/cartridges

Record hammer/safety positions and ammo locations; numbering helps reconstruct firing sequence.

Primary concern when handling bullets/cartridge cases and why be cautious removing a lodged bullet

Preserve class and individual markings; scratching can destroy striations needed for comparison.

Two types of marks giving a tool individuality and how they form

Microscopic machining marks from manufacturing and nicks/breaks from normal use.

How unremovable tool marks are analyzed and the disadvantage

Photographed and cast with silicone; some fine details may be lost.

What is done before moving any impression and why it's only backup

It is photographed thoroughly; serves as backup if the impression is damaged.

What impression evidence a forensic odontologist might analyze and how it helps identify a suspect

Bite marks on people or objects; enough matching points can link marks to one individual.

Firearms Identification

A discipline focused on determining whether a bullet or cartridge was fired by a specific weapon.

Revolver

A handgun with multiple firing chambers arranged in a revolving cylinder.

Shotgun Shell (Shot)

Shotgun ammunition containing many small spherical projectiles.

Choke

The narrowing at the end of a shotgun barrel that tightens the spread of shot.

Rifle

A long gun with a rifled barrel containing lands and grooves.

Gun Barrel Markings

Unique microscopic patterns left on bullets as they travel through the barrel.

Rifling

The process of cutting or forming spiral grooves inside a gun barrel.

Lands

The raised portions inside a rifled barrel between the grooves.

Grooves

The recessed channels inside a rifled barrel that spin to the bullet.

Caliber

The diameter of a gun barrel measured between opposite lands.

Broach

A tool that cuts all rifling grooves in one pass during barrel manufacturing.

Button

A tool that presses rifling grooves into the barrel all at once.

Hammer Forging

A method where the barrel is formed around a mandrel containing the reverse rifling pattern.

Striations

Individual microscopic lines inside the barrel that produce unique markings on bullets.

A database system storing digital images of bullets and cartridge cases to link firearms to crimes.

NIBIN (National Integrated Ballistics Information Network)

Distance Determination

Estimating the firing distance by comparing powder

Stellate Tear Pattern (star)

pattern around the bullet hole entrance, surrounded by a rim of a smokeless deposit of vaporous lead

cases where the weapon is held in contact with or less than 1 inch from the target

Vaporous Lead Smoke Ring

A halo of lead vapor around a bullet hole indicating a firing distance of 12-18 inches

Powder Particle Scatter

Unburned or partially burned gunpowder grains found around entrances up to ~25 inches

Primer Residue

Barium and antimony particles deposited on a shooter's hands after firing.

Tool Mark

An impression, cut, gouge, or abrasion produced when a tool contacts another object.

Comparison Microscope

A microscope with two viewing paths allowing side

Impressions

Marks left by footwear, tires, or fabric textures that can be photographed, lifted, or cast.

Points of Comparison

The matching individual or class characteristics used to determine whether two marks originated from the same source.