Firearms and Toolmarks in the FBI Laboratory, Part 1, by Schehl (Forensic Science Communications, April 2000)
April 2000 - Volume 2 - Number 2
Firearms and Toolmarks in the FBI Laboratory
Sally A. Schehl
Forensic Science Communications
Forensic Science Research Unit
Federal Bureau of Investigation
Introduction | Firearms Identification | Rifling | Identifying Features of Fired Ammunition | Firearms Examinations | Known Versus Questioned Specimens | Examination Results | Toolmark Identification | Disposition of Evidence | Reference Collections and Databases | Analysis and Testing of Evidence | Role of the Firearms-Toolmarks Unit in Law Enforcement
The Firearms-Toolmarks Unit (FTU) is one of many subdivisions of the FBI Laboratory devoted to a specific discipline of forensic science. This unit, comprised of firearms examiners and physical science technicians, receives and examines all incoming evidence related to firearms, firearm components, ammunition, ammunition components, tools, and toolmarks.
Forensic firearms examinations are based on firearms identification, which involves the identification of a bullet, cartridge case, or other ammunition component as having been fired by or in a particular firearm. The possibility for such singular identification can be attributed to specific machining processes used in the manufacture of firearms.
Identifying Features of Fired Ammunition
In addition to the rifling marks produced on a bullet by its passage through a gun barrel, a number of other impressions found on cartridge cases and resulting from machining processes are crucial to firearms identification. Firing pin impressions, breechface marks, extractor marks, ejector marks, and chamber marks, when present and of sufficient quality, are all features used by firearms examiners in their analyses. During the discharge of a firearm, the firing pin strikes the primer of a cartridge, creating microscopic contact marks and unique indentations. As the powder within the cartridge begins burning, the cartridge case is propelled backwards against the breechface with enough force to be impressed with the characteristic microscopic features of that surface.
Extractor and ejector marks are produced when the cartridge case is mechanically extracted from the chamber and ejected and are visible as fine striations and gouged impressions on the rim and head of the case. Chamber marks, parallel striations on the cartridge case caused by contact with the walls of the chamber of the firearm, also occur at this time. All of these potentially identifying features are produced as a result of the machining and finishing processes of firearms manufacture, which inevitably leave microscopically rough areas and edges on the parts of a given gun. During discharge, these imperfections are transferred from the metal parts of the firearm to the bullet and cartridge case.
|Firing pin impressions||Breechface and ejector marks|
Using the various microscopic signatures created by a firearm during discharge, a firearms examiner compares submitted bullets and ammunition components to each other as well as to any number of firearms.
Because a bullet, bullet jacket, or cartridge case cannot be directly compared to the rifling present in the barrel of a firearm or to the firearm’s parts, the examiner will test fire an incoming weapon into a water tank to produce known, fired specimen bullets and cartridge cases for use in comparison with questioned (evidence) ammunition components. The large volume of water contained in this tank slows a discharged bullet’s flight with no damage or distortion to the projectile and the impressions it carries, thereby generating ideal samples for microscopic examination. Fully automatic firearms, high caliber firearms, and shotguns, which cannot be discharged into the water tank, are tested on the Firearms-Toolmarks Unit’s indoor range or on outdoor firearms ranges at the FBI Academy in Quantico, Virginia.
Test firing into a water tank
|A rifle deck at Quantico|
Known, test-fired specimens are compared to questioned evidence specimens using the comparison microscope. Consisting of two separate microscopes joined by an optical bridge, this microscope allows the side-by-side observation and comparison of the microscopic characteristics present on different bullets or cartridge cases. A camera attached to this scope provides photographic documentation of these specimen comparisons, which are conducted with regard to class characteristics and individual characteristics.
In firearms identification, class characteristics include the number and direction of a barrel’s rifling (e.g., four grooves, right twist or six grooves, left twist), caliber or gauge, and the width of lands and grooves. Individual characteristics are distinct, unique marks produced during the manufacturing process and include signatures of damage and wear, such as the impression left by a deformed or broken firing pin or the unusual striations left on a bullet by a spur on a sawn barrel. These features, in combination with the microscopic marks left on bullets and cartridge cases as a product of the discharge of a firearm, enable an examiner to identify and classify ammunition components and firearms in relation to each other.
In instances of severe leading, mutilation, or corrosion of a recovered weapon, the unique microscopic markings normally present in the barrel and other portions of the firearm may be obscured or obliterated and thus may preclude identification. Conclusive identifications of bullets or other ammunition components are similarly impossible when the rifling impressions on these components match the rifling type of a given firearm, but no other distinct, unique characteristics are present on the ammunition. In other words, a bullet may bear class characteristics like those produced by the barrel of a particular type of firearm but may not be impressed with individual identifying characteristics that match it with a single specific firearm.
|A comparison microscope|
|Comparison of the breechface impressions
on two cartridge cases showing similarity
of microscopic characteristics:
a positive identification
|A microscopic identification of two
bullets showing the agreement
of class characteristics
|Additional information on the forensic examination and identification of firearms and firearms components is available in the Firearms Examinations and Elemental Analysis Examinations sections of the Handbook of Forensic Services.|