How Firearms Mark Evidence
When a firearm is manufactured, the drilling, reaming, and rifling of the barrel create microscopic imperfections unique to each weapon—like a fingerprint. When a bullet travels through the barrel, rifling (spiral grooves cut into the bore) imparts spin for accuracy and engraves the bullet with lands (raised areas) and grooves (cut areas). These marks have two levels: class characteristics (caliber, number of lands and grooves, direction of twist) that narrow the weapon to a type and manufacturer, and individual characteristics (unique microscopic striations from random tool marks during manufacturing and use) that can link a bullet to a specific firearm. Similarly, the firing pin, breech face, extractor, and ejector leave unique marks on cartridge cases.