BALLISTIC fingerprinting is one of the latest buzzwords in the fight against gun crime. Every time a gun fires a round it leaves a unique fingerprint of scratches and dents on the shell's casing. Law enforcement agencies hope to use the unique signatures to match casings found at crime scenes to fingerprints of individual weapons held in a database.

New York and Maryland have laws requiring that all new guns be fingerprinted before they are sold. And police across the US use a ballistic fingerprinting system in which digital images of bullet casings found at crime scenes are entered into a national database in the hope they can later be matched to a weapon.

But the system is not foolproof. The more fingerprints that are added to a database the more difficult it is to make an accurate match. If the system is used to create fingerprints of new weapons, ...