During the sniper shootings of 13 people in the Washington, D.C., area last October, police initially uncovered only a sparse trail of evidence--often just the bullet itself. As tension mounted over several weeks, newscasts repeatedly reported that examinations of bullet fragments were linking the shootings. That coverage brought the science and technology of bullet identification onto center stage. It also drew attention to a system still under development, in which images of bullets, bullet fragments, and bullet cases collected from crime scenes are matched against a database of previously recorded images.

Known as the National Integrated Ballistic Information Network (NIBIN), the system is already giving police a hew way to ferret out links between crimes. Law enforcement officers have used the system many times to establish connections. However, officials won't say whether it served this purpose in the Washington sniper case.

Government and industry scientists are now working on ways to fine-tune this bullet-matching system. One critical component of this effort is the creation of unfired bullet replicas that look, even on a microscopic level, like they've been shot from a gun.

Such replica bullets are needed, their developers say, to ensure consistent performance and use of bullet-identification equipment across a national network. These bullets provide, in the parlance of analysts, a reference material akin to a standard weight that can be placed on any scale. Because the bullet replicas can be duplicated with extreme fidelity and distributed to bullet-matching analysts across the country, they'll offer a uniform standard against which the analysts can calibrate their equipment and their image-recording practices.

To check the uniformity of those replicas, their developers at the National Institute of Standard and Technology (NIST) in Gaithersburg, Md., have also devised a new way to compare bullet surfaces mathematically. This method may ultimately enable forensic scientists to numerically score the degree of similarity between two bullet samples--as is already done in comparisons of DNA samples--rather than rely solely on the judgment of experts.

BULLET MUG SHOTS When forensic scientists talk about bullets, they are referring to the metal slugs that zoom through gun barrels, not the gunpowder-packed cases that hold the projectiles before the shots are fired.

Bullets are intentionally made a bit too wide to fit easily through gun barrels. That way the hard barrel compresses the relatively soft metal of the bullet as the exploding gunpowder hurls the projectile down the barrel. The compression squashes the bullet slightly, enabling about a half-dozen spiral grooves cut along the barrel's inner wall to grab the bullet and make it spin. That spin stabilizes the bullet's imminent flight.

Between the spiral grooves are the so-called lands where the barrel is thickest. Those areas, which typically have unintentional microscopic scratches on them from their manufacture, squeeze the bullet the most and leave a signature of their scratches on its surface.

To cheek whether a bullet from a crime scene was fired from a specific gun, firearms examiners typically test-fire a bullet from the suspect gun and then compare the scratches on its land impressions to those on the crime-scene bullet. Bullet cases also get nicked and dinged by a gun, so examiners often scrutinize them, too.

The technology for making such comparisons hasn't changed much since the 1920s, when firearms examiners started using so-called comparison microscopes. Those devices optically present in one eyepiece side-by-side views of two different bullets, and an examiner judges their similarity for the court report.

However, in the past decade, technology developers have created automated bullet-matching workstations that meld traditional comparison microscopes with digital cameras, lasers, computers, huge databases, and image-analysis techniques. The result: an unprecedented tool for investigators that links crimes by automatically finding similarities among images of bullets or bullet eases from crime scenes or victims. However, courts don't accept as evidence the results of an automated search without verification by a firearms examiner looking at the actual bullets or cases with a comparison microscope.

In the early 1990s, the FBI and the Bureau of Alcohol, Tobacco and Firearms (ATF) each began installing different commercial versions of such systems in their crime labs. They also started electronically linking these systems via high-speed networks to serve wider regions. Ultimately, the two agencies opted to unite their systems and equip all the labs with the same type of workstation, called the Integrated Ballistics Identification System (IBIS).