Firearms and Toolmarks in the FBI Laboratory, Part 1

• NCJ Number: 218811
• Author(s): Sally A. Schehl
• Date Published: April 2000
• Length: 5 pages
• Sale Source: US Dept of Justice Federal Bureau of Investigation
• Publication Format: Document (Online)
• Publication Type: Report (Technical)
• Language: English
• Country: United States of America
• Note: From Forensic Science Communications, V 2, N 2, April 2000; downloaded June 12, 2007.

Annotation

This description of the work of the FBI's Firearms-Toolmarks Unit focuses on firearms identification and examination.

Abstract

Firearms identification involves the identification of a bullet, cartridge case, or other ammunition component as having been fired by a particular firearm. Such distinctive identification is possible due to machining processes used in the manufacture of firearms. Because the equipment used in the machining and finishing of firearms is inherently imperfect at the microscopic level, a machined, rifled barrel will have unique scratches, scrapes, and other small nicks and flaws. These unique imperfections are transferred to the ammunition in the course of its firing, making it possible to determine which firearm fired a specific bullet. In addition to the rifling marks produced on a bullet as it travels through the gun's barrel, other impressions found on cartridge cases due to machining processes are crucial to firearms identification. These impressions include firing-pin marks, breechface marks, extractor marks, ejector marks, and chamber marks, which leave distinctive impressions on the cartridge. Using the various microscopic features created by a firearm during discharge, a firearms examiner compares submitted bullets and ammunition components to one another as well as to submitted firearms. In order to match an evidence bullet to the firearm that discharged it, the submitted firearm must be test fired into a water tank to produce fired specimen bullet and cartridge cases for comparison with the evidence ammunition components. Following the comparison examination, the examiner will draw one of three conclusions: "identification," "exclusion," or "no conclusion." "Identification" signifies a match between the evidence ammunition and the test-fired ammunition. "Exclusion" is a clear nonmatch between test and evidence ammunitions. "No conclusion" means that the quantity and quality of microscopic markings were insufficient for a conclusion.