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Firearms and Toolmarks in the FBI Laboratory, Part 1, by Schehl (Forensic Science Communications, April 2000)

Firearms and Toolmarks in the FBI Laboratory, Part 1, by Schehl (Forensic Science Communications, April 2000)

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April 2000 - Volume 2 - Number 2

Firearms and Toolmarks in the FBI Laboratory  

Part 1

Sally A. Schehl
Associate Editor
Forensic Science Communications
Forensic Science Research Unit
Federal Bureau of Investigation
Washington, DC

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


Introduction

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.

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Firearms Identification

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.

Rifling
Helical grooves known as rifling are cut into the bore of a barrel of a firearm during production to increase the accuracy of that firearm. When the gun is discharged, these grooves cause the bullet to spin as it travels the length of the barrel and thus stabilize the bullet during flight. At the same time, the expansion of the fired cartridge and the high pressures propelling the bullet through the bore of the barrel press and scrape the bullet against the rifling as it heads toward the muzzle. The fired bullet, as a result, will bear the negative impressions of the grooves in a rifled barrel; these marks are described by firearms examiners as land and groove impressions, or lands and grooves.

Because the equipment used in the machining and finishing processes of firearms production is inherently imperfect at the microscopic level, a machined, rifled barrel will contain scratches, scrapes, and other minute nicks and flaws. These unique imperfections are exacerbated through the subsequent use and discharge of the firearm, as further abrasion of the barrel occurs, and as a result of natural wearing processes such as rusting and corrosion.

A lead or jacketed bullet propelled by high pressures at great speed through the barrel of a given firearm, then, will have impressed on its surface not only the general rifling characteristics of that barrel, but also microscopic marks unique to that barrel, marks not found in the barrel or rifling of any other firearm.

cross-section of a rifled barrel
Cross-section of the barrel of a firearm showing lands and grooves 
a rifled barrel
 Rifling in the barrel of a firearm
 barrel rifling, microscopic view
Microscopic view of imperfections
in the rifling of a barrel
 rifling impressions on a bullet
Rifling on a fired bullet


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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 impression, centerfire
firing pin impression, rimfire  


breechface marks


ejector marks
Firing pin impressions   Breechface and ejector marks


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Firearms Examinations

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.

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 water tank


 Test firing into a water tank
 rifle deck
A rifle deck at Quantico 

Known Versus Questioned Specimens


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.

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comparison microscope

Examination Results
Following examination of case material, a firearms examiner will arrive at one of three conclusions: identification, exclusion, or no conclusion. An identification signifies a match between two ammunition components or a match between an ammunition component and a firearm. An exclusion represents a nonmatch between the examined items of evidence—the possibility of an association between the items is excluded. No conclusion indicates that the ammunition components could neither be identified nor eliminated as having been fired by a particular weapon based on the quantity and quality of microscopic markings. In this instance the class characteristics of the evidence in question may be in agreement, but the correspondence between individual characteristics or striae is insufficient or absent. This is not to say, however, that a given bullet could not have been fired by a submitted weapon, or from the same firearm as another bullet, but rather that the markings present on the bullet are of insufficient character to draw any conclusion.

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
breechface marks identification
Comparison of the breechface impressions
on two cartridge cases showing similarity
of microscopic characteristics:
a positive identification
identification of two bullets
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.


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Continue—Part 2