Module 4 - Forensic Ballistics

To gain a better understanding of the science behind forensics ballistics, one must understand the basic parts of a firearm and the assembly of rifle and pistol cartridges.

The Main Parts of a Firearm Cartridge

Cartridges are often referred to as bullets although this is technically an inaccurate term. A bullet is just one component of the various parts of a cartridge. A cartridge consists of a casing (2) made from brass with a primer (5) at its base (4); the casing is filled with a particular amount of gunpowder (3) on top of which sits a projectile called a bullet (1).

A Handgun Cartridge

The photograph above displays two .357 handgun cartridges. The primer mounted in the base of each bullet is displayed in the upper right photograph. The silver-coloured casing contains a hollow point bullet designed to increase stopping power. The bullet is in the brass-coloured casing is the wadcutter design intended for target practice.

A Rifle Cartridge

Rifle (or long-gun) cartridges utilize the same basic designs as handgun cartridges and can carry the same calibre bullet as a pistol cartridge, but they contain larger amounts of gunpowder in the elongated casings. As a result, muzzle velocities of bullets leaving a rifle barrel are often much higher than the muzzle velocities of handgun rounds.

The mechanics of loading and firing a cartridge are essentially the same in both types of firearms. A cartridge is initially loaded into the cylinder or magazine of the firearm. Then, it is manually loaded into the breech by cocking the weapon. This places the firing pin in a position immediately behind the primer. When the person using the firearm pulls the trigger, the firing pin is released, and it strikes the primer. When the primer is detonated, it ignites the gunpowder in the casing, causing a controlled explosion in which a high-pressure pulse of hot-gas is released to force the bullet out of the casing and down the length of the gun’s barrel. The longer the barrel, the greater the muzzle velocity because the exploding gases have more time to exert pressure on the bullet. As the bullet leaves the muzzle of the gun, a brief flash may be evident followed by smoke and the scent of gunpowder as gases escape. The empty casing usually remains in the cylinder of a revolver or rifle.

With a semi-automatic weapon, the recoil results in the used casing being ejected and a new cartridge chambered. This happens once per trigger pull. Revolvers are structured in a fundamentally different way—each pull of the trigger causes the cylinder to turn slightly. This movement lines up another chamber with the barrel of the gun.