Module 4 - Forensic Ballistics

The nature of a wound caused by a particular bullet can provide clues to help identify the type of firearm used by the suspect. Numerous factors related to the ballistic properties of a bullet can influence wound characteristics.

Kinetic Energy, Mass, Velocity, and Tumbling: As a bullet travels towards its target, it spirals through the air similar to a football thrown by a quarterback. This is due to the influence of lands and grooves in the gun barrel causing the bullet to spin as it exits the barrel. A bullet is said to tumble if it flips end over end as it approaches its target. A bullet that tumbles has more kinetic energy; therefore, it can cause a more serious injury. A light, high velocity bullet (such as a .223 rifle round) begins tumbling rapidly in tissue. This causes a pronounced cavitation or large wound resulting from the relatively large amount of kinetic energy that is transferred from the bullet to the target.

A bullet’s kinetic energy is dependent upon its mass, its velocity, and whether it tumbles as it moves. Recall the formula for kinetic energy (E_k=1/2mv² ): the faster a bullet is travelling when it strikes the target or the greater the mass of the bullet, the more kinetic energy that bullet will have and the more serious the tissue damage will be. However, if a bullet exits the target, it retains some of its kinetic energy and, therefore, causes less damage.

Bullet Design: As a bullet enters an object, the bullet expands in diameter, a process referred to as the mushroom effect. This expansion releases a bullet’s kinetic energy within an extremely short period of time and causes significant tissue damage. Hollowpoint bullets expand upon impact, especially when travelling at over 305 m/s.

Distance to Target: The distance between the muzzle of a firearm and the intended target has a large role in the kinetic energy a bullet loses during its flight. The kinetic energy lost by the bullet also depends on the type of firearm and the type of cartridge used. For example, most bullets fired from handguns lose significant amounts of kinetic energy after 100 metres; high-velocity military rifle rounds still possess considerable kinetic energy at distances of 500 metres or more. Compared to a handgun round, a rifle bullet possesses more kinetic energy over long distances because its cartridge contains more gunpowder which increases the amount of kinetic energy transferred to the bullet and therefore, increases the velocity of the bullet.

Type of Tissue: The severity of a bullet wound depends on factors such as tissue density and elasticity. Tissues that are more dense (such as bone) tend to sustain more tissue damage, This is because the bullet may fragment and/or cause fragmentation of the bone. Organs such as the liver, spleen, and kidney are relatively inelastic and are easily injured. Fluid-filled organs such as bladder, heart, large blood vessels, and intestines may rupture when struck by a bullet because of the cavitation produced by accompanying shock waves. Conversely, highly elastic tissue suffers less damage. For example, lung tissue, which is more elastic than other internal organs, usually fares better than other tissues when struck by a bullet.