To compare a bullet to a firearm, examiners first check its caliber and rifling impressions, states FirearmsID.com. Experts use a digitized system to inspect bullets and casings found at crime scenes and compare them to a large database of test-fired projectiles, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Every firearm is as unique as a fingerprint. The Integrated Ballistic Identification System compares bullet and shell casing information to the information already contained in its catalog and narrows the list of possible matches. A firearms examiner then performs a visual inspection to make a positive match to a weapon, says the ATF.
The base of a fired bullet shows irregular dimples created by acceleration pressure as it passes through the barrel of a firearm, explains the Mercer University School of Medicine's Internet Pathology Laboratory for Medical Education. The interior of the weapon's barrel also leaves rifling impressions, called striae, along the bullet's sides. These impressions can communicate imperceptible imperfections of the fired gun's barrel. The tip of the bullet, or its nose, bears information about the target that it hit.
Test-firing consists of taking bullets of the same caliber and type as the recovered bullets, making a reference mark on the bullets' tip, then firing consecutive bullets into a water tank, describes the Internet Pathology Laboratory for Medical Education. A forensic comparison microscope compares all the markings left on a crime scene bullet's surface to those on bullets test-fired in a laboratory, reports the Wisconsin State Crime Laboratory.