Bullets and Bunkum: The futility of 'ballistic fingerprinting'.

In the wake of the Washington, D.C., sniper attacks, many are viewing ballistic fingerprinting as a magic crime-solving tool. According to the pro-gun-control Brady Campaign, such a system "would have solved [the sniper case] after the first shooting"; a Washington Post columnist calls it a "common-sense measure"; and many politicians are jumping on board, including New York Democratic senator Charles Schumer and Maryland Democratic gubernatorial nominee Kathleen Kennedy Townsend. Unfortunately, the issue is not so simple: By draining resources away from other police activities and making it costly for law-abiding citizens to own guns, ballistic fingerprinting could end up actually increasing crime.

The physics of ballistic fingerprinting are straightforward. When a bullet travels through the barrel of a gun, the friction creates markings on the bullet. If the gun is new, imperfections in the way the barrel is drilled can produce different markings on the bullet; such imperfections are most noticeable in inexpensive guns. (This poses an irony for gun controllers, who push for laws that ban inexpensive guns.) In older guns, the bullets' friction through the barrel can cause more noticeable wear marks that help differentiate between guns. Many other factors influence the particular markings left on the bullets-for instance, how often the gun is cleaned and what brand of cartridge is used.

Precisely because friction causes wear, a gun's ballistic fingerprint changes over time-making it drastically different from such forensic evidence as human fingerprints or DNA. The recording of a child's fingerprints or DNA still allows for identification much later in life; the same is not true of the bullet markings. A ballistic fingerprint is less like a human fingerprint than it is like the tread on a car tire.

Brand-new tires are essentially identical, so new-tire tracks at crime scenes leave investigators with pretty limited information. Unless there happens to be a particular imperfection, only the brand and model of the tire can be identified. Imprints on bullets are similar. When a bullet is fired from a new gun, investigators can typically identify only the type of ammunition and the type of gun. Over time, though, friction causes the tread on tires to wear. It would be easy to take the tire tracks left at a crime scene and match them with a suspected criminal's car; but the more the car is driven after the crime, the harder it is to match the tire tracks left at the scene to the tires when they are eventually found. Similarly, the greatest friction on a gun occurs when the gun is first fired-and that dramatically reduces the usefulness of recording the gun's ballistic fingerprint when it is purchased.

Moreover, ballistic fingerprinting can be thwarted by replacing the gun's barrel-just as criminals can foil tire-matching by simply replacing their tires. In general, the markings on bullets can be altered even more quickly and easily than the tread marks on tires: Scratching part of the inside of a barrel with a nail file would alter the bullet's path down the barrel and thus change the markings. So would putting toothpaste on a bullet before firing it.

Ballistic fingerprinting faces other difficulties. For example, even if the gun was not used much between the time the ballistic fingerprint was originally recorded and the time the crime occurred, police still have to be able to trace the gun from the original owner to the criminal-but only 12 ...