Module 4 - Forensic Ballistics

In any criminal investigation where a firearm has been used, evidence may be analyzed, which includes the weapon, a bullet, a piece of the bullet (fragment), a cartridge casing, bullet holes, the injury caused by the bullet, or even gunpowder residue. All these clues can be used to match a suspect weapon to a crime.

Ballistic Fingerprinting

Ballistic fingerprinting involves the identification of unique markings on bullets and cartridge casings that have been located at crime scenes. By comparing the striations and marks left on a bullet or shell casing recovered at a crime scene to those left on a bullet or shell casing by a gun that has been test fired in a laboratory, police can determine whether that specific firearm was involved.

Land and Groove Impressions

The interior of the barrel in every gun contains unique striations called lands and grooves. The lands are raised ridges, and the grooves are recessed portions between each of the ridges. Known as rifling, these lands and grooves are cut into the barrel during production to increase the accuracy of that firearm. When the gun is discharged, these lands and grooves cause the bullet to spin as it travels the length of the barrel. This stabilizes the bullet during flight. At the same time, the expansion of the fired cartridge and the high pressure propelling the bullet through the barrel press and scrape the bullet against the rifling as it moves toward the muzzle. As a result, the fired bullet has unique microscopic striations left upon its exterior from the lands and grooves. These striations are called land and groove impressions by forensic firearm investigators.

Because rifling is randomly generated during manufacture or due to wear, no two gun barrels leave the same land and groove impressions upon a bullet. This uniqueness enables the identification of bullets as having originated from a particular gun.

To link a bullet to a particular firearm, forensic ballistics experts typically fire a crime scene weapon into a tank of water and then retrieve the test bullet. The test bullet is then compared to the bullet in question using a comparison microscope. If the land and groove impressions match, a photograph is taken for use as evidence in court. However, if a weapon is not left at the crime scene and the crime scene investigators are able to retrieve an intact bullet from the victim or the scene, they can only make assumptions about the type and model of firearm used based on the calibre of the bullet and the wound ballistics. Simply based on the rifling impressions on a particular bullet, it would be impossible for investigators to deduce the type and model of firearm because rifling impressions are unique for every gun not for every type of gun (for example, all .44 magnum pistols will not have similar rifling impressions). The only exception would be if the same gun had been used in a previous crime and both the bullet and the gun had been entered into the ballistic's database.