Firearms and Toolmarks

Abstract

This chapter covers the entire scope of firearm and toolmark examination, from the scientific basis for identification and the general protocol used, to the related duties that are involved in the daily routine of the examiner. Specific case examples are utilized to illustrate the various topics discussed.

Glossary

Annular rim

The outer circumference of the cartridge case base (location of priming mixture in rim fire cartridges).

Antimony sulfide

A component of most priming mixtures that acts as a fuel.

Ballistics

The study of projectile motion, often confused with firearms identification.

Barium nitrate

A component of most priming mixtures that acts as an oxidizer.

Bird shot

A general term for any shot smaller than buckshot.

Bolt action

A firearm in which the breech is always in line with the bore and manually reciprocates to load, unload, and cock (two principle types rotating and straight pull).

Bore

The interior of a barrel forward of the chamber.

Breech face

That part of the breech block or bolt that is against the base of the cartridge case or shot shell during firing.

Broach

Rifling cutter that cuts all the grooves simultaneously.

Buckshot

Lead pellets ranging in diameter from 0.02 to 0.36 in. and normally fired in shotguns.

Bullet

The projectile portion of a cartridge.

Bullet jacket

Metallic covering over bullet core.

Caliber

The cross-sectional diameter of the barrel from land to land.

Calipers

A device consisting of two moveable jaws or legs used to measure distance, thickness, or width.

Cartridge

Ammunition component consisting of a cartridge case, bullet, powder charge (propellant), and primer.

Cartridge case

The container for all the other components of a cartridge.

Center fire

Cartridge with the primer in the center of the base (head).

Chamber

The rear part of the barrel bore that has been machined for a specific cartridge (revolver cylinders are multi-chambered).

Chamber marks

Individual characteristics imparted to the chamber walls during machining.

Choke

Constriction in the muzzle end of a shotgun barrel.

Class characteristics

Those characteristics exhibited by an entire class or group.

Clip

A separate device for magazine reloading.

Cock

To place a firing mechanism under spring tension.

Disk powder

An extruded form of gunpowder that is cut into small disks.

Double action

A single pull of the trigger cocks and releases the hammer.

Ejection

The expulsion of a fired cartridge case or shot shell.

Ejector marks

Marks left on the base (head) of a cartridge case or shot shell by the ejector during the process of ejection.

Etch

To produce corrosive action on metal.

Falling block

A single-shot lever action mechanism in which the breech block slides vertically or nearly vertically down as the lever is worked.

Firing pin

That part of a firearm mechanism that strikes the primer and initiates ignition.

Firing pin impression

The impression left by the firing pin upon impact with the primer.

Function testing

The examination of a firearm for operability and firing capability.

General rifling

The number, width, and direction of twist of rifling grooves.

Griess test

Chemical test for nitrites used to detect gunpowder residue around bullet holes.

Grooves

Helical grooves in the interior of the barrel to impart spin on the bullet.

Gunshot residue

Gunpowder and primer residue resulting from discharge.

Hammer

That part of the firing pin that imparts energy to the firing pin.

Hammer forging

Process of forming the interior/exterior of a barrel by hammering.

Individual characteristics

Accidental, random marks used to identify toolmarks.

Lead azide

Chemical compound used in most priming mixtures.

Lever action

Type of firearms action that utilizes a lever to move the breech mechanism.

Micrometer

Precision measuring device used to measure small distances/thickness.

Nitrocellulose powder

A smokeless propellant whose principle ingredient is nitrocellulose.

Nitroglycerin

A high explosive and component of double-based gunpowder.

Pellet

Common term for small, spherical shot used in shot shells.

Polygonal rifling

Rifling with rounded edges instead of the usual square edges.

Powder stippling

The result of powder particles striking the skin and imbedding and/or leaving a burn or bruise.

Primer

Shock-sensitive explosive mixture that initiates burning of the propellant.

Propellant

The powder charge inside a shot shell or cartridge case.

Slug

Single projectile for a shotgun.

Sodium rhodizonate

Chemical test for lead.

Toolmark, impression

The result of a tool pressed against another surface with enough force to leave and impression.

Toolmark, striated

A mark produced with a combination of force and motion.