sniper attacks in 2002—the question has been raised of expanding the scope of databases like NIBIN to include images from test firings of newly manufactured firearms. In concept, this would permit downstream investigators who recover a cartridge case or bullet at a crime scene to identify the likely source firearm. Though two states (Maryland and New York) instituted such reference ballistic image databases for newly manufactured firearms, proposals to create such a database at the national level did not make substantial progress in Congress. A recent report of the National Academies, Ballistic Imaging, examined this option in great detail and concluded that “[a] national reference ballistic image database of all new and imported guns is not advisable at this time.”60
In both firearm and toolmark identification, it is useful to distinguish several types of characteristics that are considered by examiners. “Class characteristics” are distinctive features that are shared by many items of the same type. For example, the width of the head of a screwdriver or the pattern of serrations in the blade of a knife may be class characteristics that are common to all screwdrivers or knives of a particular manufacturer and/or model. Similarly, the number of grooves cut into the barrel of a firearm and the direction of “twist” in those grooves are class characteristics that can filter and restrict the range of firearms that match evidence found at a crime scene. “Individual characteristics” are the fine microscopic markings and textures that are said to be unique to an individual tool or firearm. Between these two extremes are “subclass characteristics” that may be common to a small group of firearms and that are produced by the manufacturing process, such as when a worn or dull tool is used to cut barrel rifling.
Bullets and cartridge cases are first examined to determine which class characteristics are present. If these differ from a comparison bullet or cartridge, further examination may be unnecessary. The microscopic markings on bullets and cartridge cases and on toolmarks are then examined under a comparison microscope (made from two compound microscopes joined by a comparison bridge that allows viewing of two objects at the same time). The unknown and known bullet or cartridge case or toolmark surfaces are compared visually by a firearms examiner, who can evaluate whether a match exists.