Since the 1930s, police investigators have studied bullets to find key evidence. At a crime scene, investigators would retrieve any bullets and cartridge cases, and take them to police labs. There, firearms examiners would place the bullets and cartridge cases under a special microscope to compare the marks left by the firing gun on each. They would take snapshots and distribute them to local police departments. When police recovered a gun, they could discharge it to compare the markings left on bullets and casings against those in that collection of snapshots.Technology changed all that about a decade ago. Now, in communities across the country, the ballistics imaging and matching process is computerized. Investigators still collect evidence at crime scenes, but software programs analyze ballistics images and store the results in databases. And databases from local and regional jurisdictions are linked together in the National Integrated Ballistics Identification Network (NIBIN). But the entries in this network of databases cover only guns seized by police. This ability to analyze ballistics became a topic of interest recently, when a string of sniper shootings terrorized citizens around Washington, D.C., Maryland and Virginia before authorities caught and charged two suspects in the case. (It turns out that Maryland, along with New York, has created a statewide database for handguns owned there but not for rifles, which was the type of gun used in the sniper attacks.)Now, some are calling for the creation of a national ballistic fingerprint system that would enable police to trace bullets recovered from shootings in all states. Such a proposal is part of the gun control policy debate. But, politics aside, we wondered how technically difficult such a project would be, given potentially millions of gun records.Not very difficult, says Joe Vince, a former chief of the crime guns analysis branch of the Bureau of Alcohol, Tobacco and Firearms (ATF). "Human fingerprints were once kept on file cards, and now there is a national electronic system," says Vince, who is now president of Crime Gun Solutions, a consulting company in Frederick, Md. "It should be no different for ballistic fingerprints." Vince says such a national system would be based on a computer imaging product known as IBIS (Integrated Ballistic Identification System), which the ATF has used since 1993. IBIS, developed by Forensic Technology of Cote St.-Luc, Quebec, digitally captures images of bullets and cartridge cases, stores them in a database, performs automatic computer-based comparisons of the images and ranks them according to the likelihood of a match. Firearms examiners can then perform microscopic comparisons of the likely candidates. The IBIS system uses Oracle as its relational database management system. The system could expand to store all guns sold in the nation, says Serge Labrecque, one of the developers for IBIS. "Right now IBIS is a reactive product, taking an image once a crime has occurred" and evidence is recovered, Labrecque says. He adds that IBIS could support more information and images, with records received directly from gunmakers, before they sell or distribute the weapons.Straightforward? Not so fast. Hurdles include getting local, state and federal police agencies to cooperate, says Wayne Eckerson, research director for The Data Warehousing Institute. "Modeling the database is the easy part," he says. "The challenge will be bringing together data owned by multiple agencies."