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Is a Match Really a Match?
A Primer on the Procedures and Validity
of Firearm and Toolmark Identification

Federal Bureau of Investigation


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

The examination process typically begins when an examiner receives a suspect firearm, along with bullets (the projectiles) and spent cartridge cases recovered from a crime scene. The gun is test-fired in order to recover bullets and cartridge cases that serve as controls, or better, comparison samples. That is, these test-fired specimens will be compared at multiple levels with the specimens submitted from the crime scene.

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 >

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This Month's Featured Resource on the Crime Scene Investigator Network Website

ATF Police Officer's Guide to Recovered Firearms

U.S. Department of Justice
Bureau of Alcohol, Tobacco, Firearms and Explosives

This guide is designed for Police Officers and includes the following topics: Firearms Safety, Required Markings, How To Identify Firearms, Tips For Firearms Queries Using Mobile NCIC Terminals, Trace All Recovered Firearms, Persons Prohibited by Federal Law from Possessing Firearms,Questions to Ask Unlawful Firearms Possessors, and Visual Guide to Firearms.

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