Firearms and Toolmark Definitions

Auxiliary Chamber

An adaptation to a firearm to enable the firing of smaller or lower-powered ammunition. Sometimes called an adapter, auxiliary cartridge, or sub-caliber device.

Borescope

An instrument usually consisting of a tube, mirror, light source, and lens used to examine the bore and/or chamber of a firearm.

Caliber-

1) A term used to designate the specific cartridge for which a firearm is chambered.

Brass

1) An alloy principally composed of copper and zinc in varying proportions, often used in the manufacture of cartridge cases, primer cups and bullet jackets. 2) A slang term sometimes used to refer to fired cartridge cases.

Blowback

1) In ammunition, a leakage of gas rearward between the case and the chamber wall from the mouth of the case. 2) In firearms, a design found in semiautomatic and automatic firearms where the weight of the bolt, the inertia of some component, and/or a mechanical delay is the main locking force and no mechanical locking of the breech occurs. These firearms directly utilize the breech pressure exerted on the head of the cartridge case to actuate the mechanism.

Bore Diameter

1) In rifled barrels, it is the distance across the center of the barrel from the top of one land to the top of the opposite land. 2) In shotguns or muskets, it is the distance across the center of the barrel forward of the chamber but before any restrictive choke or expanded muzzle.

Unload

1) The complete removal of all ammunition from a firearm and/or magazine. 2) The disassembly of a cartridge or shotshell.

Body (Case)

1) The metallic portion of the cartridge case that contains the propellant, primer and projectile(s). 2) The tubular section of a shotshell that contains the propellant, wads, and projectile(s).

Armor Piercing Bullet

A bullet consisting of a hardened core or wholly composed of a substance other than lead or lead alloy.

Blank

A cartridge without a projectile designed to make noise.

Assault Rifle

A compact, select-fire, detachable box magazine-fed firearm, that utilizes a centerfire rifle cartridge and is designed primarily for military use.

Wad

A felt, paper, cardboard or plastic component used in a shotshell for various purposes. (See AFTE Glossary 6th Edition for the various types of wads).

Battery Cup

A flanged metallic cup used in shotshell primer assemblies that provides a rigid support for the primer cup and anvil. Also known as a battery pocket.

Vernier Caliper

A measuring instrument having a fixed jaw, a sliding jaw, and a point that slides along an attached scale.

Alloy

A mixture of two or more metals fused or melted together to form a new material. For example, a brass is an alloy composed of copper and zinc.

Anvil Marks

Microscopic marks impressed on the forward face of the rim of a rimfire cartridge case as it is forced against the breech end of the chamber by the firing pin. These marks are characteristic of the breech under the firing pin and have been used to identify a cartridge case to a specific firearm.

Ammunition

One or more loaded cartridges consistent of a primed cartridge case, propellant, and with or without one or more projectiles. Also referred to as fixed ammunition or live ammunition.

Assault Weapon

Slang term to describe any firearm with a military appearance or large magazine capacity.

Barrel

That part of a firearm through which a projectile or shot charge travels under the impetus of powder gasses, compressed air, or other like means. A barrel may be rifled or smooth. (See AFTE Glossary 6th Edition for the various barrel types).

Bearing Surface

That portion of a bullet's outer surface that comes into direct contact with the interior surface of the barrel.

Trigger Pull

The amount of force which must be applied to the trigger of a firearm to cause sear release. It is measured by hanging weights or an instrument touching the trigger at a point where the trigger finger would normally rest. The force applied during measurement is approximately parallel to the bore axis.

Yaw

The angle between the longitudinal axis of a projectile and the line of the projectile's trajectory. Yaw is usually considered to exist before a bullet achieves full gyroscopic stability.

Battery

The condition in a firearm action in which the bolt or breechblock is solidly secured in a fixed relationship with the chamber so as to resist being blown back by chamber pressure. Refer to In Battery and Out of Battery.

BB

The designation of spherical shot having a diameter of .180" used in shotshell loads. The term BB is also used to designate steel or lead air gun shot of .175" in diameter. Although the two definitions cause some confusion, they have co-existed for many years.

Twist

The direction of turn of the rifling helix, such as left hand or right hand twist.

Backstrap

The exposed metal strip at the rear of a pistol or revolver grip.

Best Known Non-Matching Agreement in Toolmarks

The greatest individual corresponding agreement between two toolmarks known to have been created by different tools that has either been personally observed by trained examiners or has been observed by others in the profession by rigorous studies.

Bolt

The locking and cartridge head support mechanism of a firearm that operates in line with the axis of the bore. Also known as the breech bolt. In revolvers, bolt is sometimes used to refer to the cylinder stop.

Breech

The part of a firearm at the rear of the bore into which ammunition components are inserted.

Yoke

The part of a solid frame revolver on which the cylinder is swung out to the side to accomplish loading and ejecting. Also known as a crane.

Velocity

The rate at which an object changes its position over time (V=∆d/∆t where V = velocity, ∆d = change in distance and ∆t = change in time). Velocity is a vector quantity that has both magnitude and direction.

Ballistics

The science of projectiles in motion. Usually divided into three parts: 1) Interior Ballistics, which studies the projectile's movement inside the firearm; 2) Exterior Ballistics, which studies the projectile's movement between the muzzle and the target; and 3) Terminal Ballistics, which studies the effect of a projectile's impact at the target. A term often confused with Forensic Firearms Identification.

Theory of Identification as it Relates to Toolmarks-

The theory of identification as it pertains to the comparison of toolmarks enables opinions of common origin to be made when the unique surface contours of two toolmarks are in 'sufficient agreement.'

Action

The working mechanism of a firearm. The combination of the receiver or frame, the breech bolt, and the other parts of the mechanism by which a firearm is loaded, fired and unloaded. (See AFTE Glossary 6th Edition for the various action types).

Sufficient Agreement

This 'sufficient agreement' is related to the significant duplication of random toolmarks as evidenced by the correspondence of a pattern or combination of patterns of surface contours. Significance is determined by the comparative examination of two or more sets of surface contour patterns comprised of individual peaks, ridges and furrows. Specifically, the relative height or depth, width, curvature and spatial relationship of the individual peaks, ridges and furrows within one set of surface contours are defined and compared to the corresponding features in the second set of surface contours. Agreement is significant when the agreement in individual characteristics exceeds the best agreement demonstrated between toolmarks known to have been produced by different tools and is consistent with agreement demonstrated by toolmarks known to have been produced by the same tool. The statement that 'sufficient agreement' exists between two toolmarks means that the agreement of individual characteristics is of a quantity and quality that the likelihood another tool could have mad the mark is so remote as to be considered a practical impossibility.

Bore

the interior of a barrel forward of the chamber.