Ballistic fingerprinting refers to a set of forensic techniques that rely on marks that firearms leave on bullets to match a bullet to the gun it was fired with.
Ballistic fingerprinting techniques are based on the principle that all firearms have inevitable variations due to marks left by the machining process, leaving shallow impressions in the metal which are rarely completely polished out. Also, normal wear and tear from use can cause each firearm to acquire distinct characteristics over time.
When a bullet is fired through a rifled barrel, the raised and lowered spirals of the rifling etch fine grooves called “striations” into the bullet. These can be matched with the barrel through which the bullet was fired. Examiners distinguish between striations common to all guns of a particular type (“class characteristics”) and those unique to a particular gun (“individual characteristics”).
Marks on the cartridge case can be matched to marks in the chamber and breech. For a number of reasons, cartridge cases are often easier to identify than bullets. First, the parts of a firearm that produce marks on cartridge cases are less subject to long-term wear, and second, bullets are often severely deformed on impact, destroying much of the markings they acquire.
Ballistic fingerprinting of bullets does not work with firearms such as shotguns that fire shot-containing cartridges. In many cases the shot rides inside a plastic sleeve that prevents it from ever touching the barrel, and even in cases where the shot does touch the barrel, the random movement of the shot down the barrel will not leave any consistent marks. But shotgun cases can still be examined for firing pin marks and the like.