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To evaluate old bullets, ballistics experts rely on the same basic techniques used at police labs. Most clues are found in two types of tiny markings, called class and individual characteristics.
By looking for the bullet caliber and “rifling” marks, experts can identify the type of gun used. Rifling marks are caused by spiral grooves located inside the gun barrel. These spirals cause the bullet to spin, producing a more stable flight path.
Each type of gun (for example a .38 Smith and Wesson or a Colt .45) is manufactured with a distinctive rifling pattern, turning to the right or left, at a specific rate of twist.
When a gun is fired, small imperfections inside the barrel leave a unique pattern of marks on the bullet.
Two bullets fired from the same gun will bear identical individual characteristics. If a ballistics examiner has access to the weapon that allegedly fired an old bullet, new “test” bullets can be fired and compared to the old bullet.
To look for matching marks, the bullets are examined under a special microscope that allows side-by-side comparison of two objects. (In fact, the comparison microscope, now used in many fields of science, was invented in 1923 specifically for the purpose of comparing bullets.)
But not all bullets are easily identified. Small-caliber bullets have fewer identifiable characteristics than larger bullets.
Bullets fired before the late 18th century can be especially difficult to identify. In those days, pistols and muskets had “smoothbore” barrels, which left no rifling marks.