Chapter 21: Firearms and Tool Marks

Firearms examination

one of the key services a forensic science laboratory provides; even smaller laboratories with cooperation or direct harm, and society has judged this implied or actual violence to be a severe crime.

Ballistics

the study of an object in flight and is under the domain of physics.

Handgun

designed to be fired with one hand.

Revolver

a handgun that feeds ammunition into the firing chamber by means of a revolving cylinder.

(Semi)Automatic Pistol

feeds ammunition by means of a spring-loaded vertical magazine.

Shoulder Firearms

more diverse, encompassing rifles, shotguns, machine guns, and submachine guns.

Rifles

esigned to be fired from the shoulder with two hands; it may be single-shot, repeating, semi-automatic, or automatic.

Single-shot rifle

must be loaded, fired, the cartridge extracted, and then reloaded.

Repeating rifles fire

one bullet with each pull of the trigger, but the expended cartridge must be expelled, the firing mechanism cocked, and a live round reloaded from a magazine manually.

Semiautomatic rifles

use the energy of the fired ammunition to expel the empty cartridge, cock the firing mechanism, and reload a live round; thus, one pull of the trigger fires one round, and this may be done sequentially until the magazine is empty.

Assault rifles

can be fired either like semi-automatic rifles or in automatic mode: Pull the trigger, and the firearm will fire ammunition continuously until all the ammunition is gone

Machine gun

a fully automatic firearm and therefore is fed ammunition from a high-capacity belt or box. Because of their size and the strength of the recoil, machine guns are meant to be fired from a tripod or other mounted/fixed position.

Submachine gun

a machine gun meant to be fired while held in the hands.

Lands and Grooves

series of ridges and valleys rifled in the interior surface of the barrels of the firearms.

Striations

microscopic contour variations on the surface of the bullet

Broach or Rifling Button

stiff metal rod with a flanged tip, which is run down the length of the hole.

Bore diameter

the diameter of a circle that touches the tops of the lands.

Caliber

refers mostly to the size of a particular ammunition cartridge.

Pellets

numerous projectiles a shotgun fires.

Slugs

single projectiles.

Choke

helps to keep the pellets grouped longer once they leave the barrel.

Gauge

diameter of the shotgun barrel.

Ammunition

a self-contained cartridge that is composed of one or more projectiles, propellant (to act as fuel), and a primer (to ignite the propellant).

Lead bullets

pieces of lead hardened with minute amounts of other metals and formed into the desired shape.

Fully jacketed cartridge

a lead core that is encased in a harder material, usually copper–nickel alloys or steel.

Semi-jacketed cartridge

has a metal jacket that covers only a portion of the bullet, with the nose often exposed.

Hollow-point cartridges

a semi jacketed bullet that has a hollowed-out tip to increase this effect.

Propellant

the fuel that propels the projectile down and out of the firearm’s barrel.

Black Powder

the first propellant to be used in firearms.

Smokeless Powder

Composed of nitrocellulose combined with various chemicals to stabilize the mix and modify it for safe manufacture and transport.

Primer

what ignites the propellant. It consists of a small metal cup containing a percussion-sensitive material that, when struck, creates enough heat to ignite the propellant.

Recoil

causes the cartridge base to smack against the breech face and receive an impression of any imperfections in the breech face.

Firing Pin Impression

the mark made by the firing pin as it strikes the printer cap.

Comparison stereomicroscope

used in performing bullet comparisons.

National Integration Ballistic Information Network (NIBIN)

automates ballistics evaluations and provides investigative leads by revealing connections between crimes in which the same firearm is used.

Primer Residues

mostly microscopic blobs of the molten metals

Sodium Rhodizonate

last chemical treatment used to visualize GSR

Bullet wipe

Residue of lead, primer materials, carbon, and other materials from the barrel that are transferred onto the outermost surface of the target by the bullet as it passes through.

Gunpowder Residues

violent chemical reaction of the primer and accelerant results in a cloud of molten metals, partially burned gunpowder flakes, smoke, and other microscopic debris.