Abstract

The science of firearm and toolmark identification has been a core element in forensic science since the early 20th century. Although the core principles remain the same, the current methodology uses validated standard operating procedures (SOPs) framed around a sound quality assurance system. In addition to reviewing the standard procedure the FBI Laboratory uses to examine and identify firearms and toolmarks, we discuss the scientific foundation for firearm and toolmark identification, the identification criterion for a "match," and future research needs in the science.

What Does an Examiner Actually Do in a Firearm Examination? The General Examination Procedure for Bullets

To our knowledge, the vast majority of forensic laboratories in the United States and abroad have SOPs (technical protocols) that set forth in detail the proper examination procedures, and these procedures are highly similar across laboratories. This similarity certainly holds for bullets, for example, the examination of which we discuss below.

The initial bullet analysis (level one) involves the assessment of firearm class characteristics. These characteristics are permanent, measurable feature(s) of a tool or firearm that were predetermined before manufacture and that establish a restricted population or group source. The analysis of class characteristics (both for barrels and the transfer of their characteristics to bullets) creates the foundation for the firearm and toolmark examination procedure. If the comparison between an evidence bullet from a crime scene and a test-fired bullet from a suspect firearm reveals a convergence of discernable class characteristics, then the suspect firearm remains within the population (group) of "suspect" firearms and the analysis moves on to level two. In the FBI Laboratory, if even one type of class characteristic shows significant divergence, then the evidence bullet and suspect firearm exist as two separate populations (groups) and the bullet is eliminated from consideration as having been fired by the suspect firearm.

What class characteristics pertain to bullets? In addition to the diameter of the bullet, class characteristics stem directly from the nature of rifling, the longitudinal grooves that run the length of and "twist" around the inside of a barrel's interior (the bore). Along with the lands, which simply are the raised portions of the bore adjacent to the grooves, the grooves are designed to impart a spin to a fired bullet so the bullet flies true, much as a quarterback imparts spin to a football when throwing a forward pass.

Generally, a firearm has three rifling characteristics: the number of land and groove impressions imparted to the bullet by the rifling in the bore, the widths of these land/groove impressions, and the direction of twist of these impressions. These characteristics are observed under a stereomicroscope. If, for example, bullet "A" displays four land/groove impressions and bullet "B" displays six land/groove impressions, then these two bullets were not fired from the same barrel. An incompatibility in any one of the three common rifling-characteristic measurements results in an exclusion.

< read the complete article and view example photographs >

• Firearm Identification in the Forensic Science Laboratory, by Robert M. Thompson
• Firearms and Toolmarks Overview, Georgia Bureau of Investigation
• How ATF Is Working to Improve NIBIN, by John Risenhoover
• The Science Behind Firearm and Tool Mark Examination, by Nancy Ritter
• Study Identifies Ways to Improve ATF Ballistic Evidence Program, by Nancy Ritter