Module 4 - Forensic Ballistics

Bolt-action rifles are reliable and easy to maintain. These rifles vary widely in calibre and purpose. They are characterized by a handle (bolt) on the side that allows the user to extract a spent cartridge and place a new one into the barrel from the magazine by sliding the bolt back and then forward.

Bolt-action rifles typically hold up to five cartridges and must be reloaded after each round is fired. Telescopic sights help bring targets into focus over long distances. Depending on its calibre, the use of a telescope mounted along the barrel above the action of the rifle increases the effective range of a bolt-action rifle to between 200 metres and 1000 metres (1 kilometre).

Because of the longer shell casings, bolt-action rifle bullets possess extremely high muzzle velocities and kinetic energies. For example, while a standard .40 calibre pistol round commonly used by police agencies has a muzzle velocity of 305 metres/second and 542 Joules of kinetic energy, a .308 round from a police sniper’s bolt-action rifle has a muzzle velocity of over 792 m/s and almost 3660 J of kinetic energy! In other words, a rifle bullet is smaller in diameter, travels faster, and has much more kinetic energy than a bullet fired by either a revolver or a pistol.

Just as muzzle velocity, kinetic energy, and trajectory vary with revolver and pistol ammunition, they also vary widely depending on the calibre of the rifle bullet. However, the trajectory of rifle bullets can in some cases be much flatter than that of handgun ammunition. For example, a handgun round fired parallel to the ground might fall into the dirt after it has travelled the length of two football fields. A rifle bullet used in big game rifles might travel over 3 kilometres before doing so.

The Automatic Rifle

Typically referred to as assault rifles, an automatic rifle is characterized by its ability to fire a continuous stream of bullets with each pull of the trigger. The rifle pictured above is a version of the standard weapon used by the US military and increasingly by police agencies across North America.

Assault rifles are highly accurate up to approximately 200 metres. They are designed for combat scenarios in which a large number of bullets are directed at a target in a brief time. Automatic weapons such as the AR-15 have been modified for law enforcement use and are valued for their accuracy and stopping power.

Automatic weapons, whether machine guns, assault rifles, or submachine guns, are available for various sizes of ammunition, including .223, 9 mm, and 7.62 mm. Most of these ammunition rounds have relatively flat trajectories. This means that a bullet can travel a great distance without falling into the ground. For example, the .223 round commonly used in the AR-15 rifle might drop only 1 to 2 cm as it travels the length of two football fields. Consequently, automatic weapons are highly accurate under controlled conditions.

Rifle Cartridges

The Shotgun

Shotguns were designed to spray lead pellets over a large area. They are ideal for hunting birds or small animals that move quickly. Shotguns are very popular due to their versatility and durability. The 12-gauge Remington 870 shotgun has been issued in law enforcement agencies for decades.

Shotguns fire shells rather than cartridges from a magazine holding three or four rounds. They can fire a wide variety of shells in various calibres. Birdshot typically consists of a large number of small lead pellets packed within the shell, which sits above a brass casing and primer. Buckshot consists of a smaller number of lead pellets, each of which is larger in diameter than birdshot. Typical buckshot rounds used by police agencies contain approximately nine lead pellets, each the size of a small pea. Its effective range is limited to 20 to 30 metres. Rifle slugs are also used in shotguns, increasing the effective range of a shotgun to approximately 100 metres (the length of a football field). Slugs are lead bullets about the size of a man’s thumb. They have substantial amounts of kinetic energy over short distances. However, they rapidly lose height after approximately 100 metres. Therefore, shotgun slugs are inaccurate past that distance.