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Firearms and Toolmarks in the FBI Laboratory, Part 3, by Schehl (Forensic Science Communications, April 2000)


April 2000 - Volume 2 - Number 2

Firearms and Toolmarks in the FBI Laboratory

Part 3

Sally A. Schehl

Associate Editor

Forensic Science Communications
Forensic Science Research Unit
Federal Bureau of Investigation
Washington, DC


Read about …

Firearms Identification
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


Analysis and Testing of Evidence

Evidence in a typical case may include a number of recovered rifles, pistols, shotguns, silencers and other muzzle attachments, magazines, holsters, and a variety of fired and unfired cartridges. Lead and other metal fragments, shot wads, shot cups, and bullets removed from bodies at autopsy are also frequently received items in firearms-related casework. Evidence submitted in toolmark cases may include screwdrivers, scissors, knives, pliers, wrenches, crowbars, hammers, saws, wire, sections of sheet metal, chains, safety-deposit boxes, human bone or cartilage, plates, locks, doorknobs, bolts, and screens.

In addition to the preliminary examinations and research conducted by technicians and the microscopic analyses performed by examiners, evidence in the Firearms-Toolmarks Unit may undergo a variety of testing. Depending on the specifics of the case and the examinations requested by a contributor, an examiner-technician team may employ the following tests and procedures:

miscellaneous firearms evidence
Firearms evidence: cartridges,
casings, and bullets
The presence of gunpowder on items
of evidence can be determined through gunshot residue


  • Trigger pull tests, which determine the amount of force needed to discharge a firearm;
  • Function tests, which determine whether a firearm is fully and normally operational in all its modes of fire and whether all safeties are operating properly;
  • Full-auto conversion tests, which determine whether a semi-automatic firearm has been modified to fire automatically;
  • Accidental discharge tests, which determine whether a firearm can fire without direct application of pressure on the trigger;
  • Silencer testing, which determines whether certain muzzle attachments can be classified as sound suppressors;
  • Gunshot residue examinations, which determine whether gunpowder or lead residues have been deposited on clothing and other items through physical and chemical examination;
  • Ejection pattern testing, which defines the position of a shooter based on the location of spent cartridge cases or shotshell casings in crime scene reconstructions;
  • Shot pattern examinations, which assist in the determination of the distance of a shooter from a target through analysis of the change in spread of shot pellets following the discharge of a shotgun;
  • Trajectory analysis examinations, which determine the angle and direction of fire and the location or positioning of a shooter through the geometric measurement and analysis of bullet holes; and
  • Serial number restorations, specialized toolmark examinations that assist in the restoration of obliterated or altered serial number characters on firearms, vehicles, or other equipment through the application of chemical, thermal, and magnetic techniques to the altered surfaces.

Toolmark examinations may also require an examiner to make casts of stamped numbers or impressions. These are made by creating a clay dam around the toolmarked area and pouring an epoxy or plastic casting material into the isolated region. Although some casts are made solely for reference purposes, the quality of detail obtained with this procedure is such that once a cast has set and dried, it can be examined and compared microscopically with other toolmark casts and questioned tools. For more information on casting techniques, see the Shoeprint and Tire Tread Examinations and Serial Number Examinations sections of the Handbook of Forensic Services.

die stamp
The Firearms-Toolmarks Unit examines Vehicle Identification Numbers (VINs; above right) and other stamped serial numbers for the presence of criminal alterations. Die stamps, such as the X character shown above left, and other equipment used to produce identifying characters may be examined in serial number restoration cases.


Role of the Firearms-Toolmarks Unit in Law Enforcement

Firearms-Toolmarks Unit personnel have contributed to investigations surrounding such high-profile cases as the assassination of Martin Luther King, Jr., the assassination of John F. Kennedy, the attempted assassination of Ronald Reagan, Ruby Ridge, Waco, the Oklahoma City Bombing, and the Centennial Park Bombing, but the majority of casework in the FTU involves what might be considered to be more typical crimes. With regard to any case, however, filing the final report and mailing back the evidence do not necessarily represent the end of the Firearms-Toolmarks Unit’s involvement in federal or local investigation. Firearms examiners are often called upon to testify to the results of their examinations as expert witnesses in criminal proceedings. Physical evidence ranging from firearms and ammunition components, tools and toolmarked items, obliterated serial numbers, silencers, and gunshot residue may confront these forensic specialists in a court of law. Though FTU examiners may serve as expert witnesses for either the prosecution or the defense, the FBI prohibits the testimony of its examiners in cases where other expert firearms- or toolmarks-related testimony is already being or has been provided.

Firearms examiners provide field support in FBI investigative matters and administrative inquiries, directing or assisting with the collection, preservation, and processing of evidence at crime scenes. In addition, field teams from the Firearms-Toolmarks Unit provide crime scene reconstructions and analyses as needed, stateside or internationally, with designated response teams on standby rotation in the event of disaster in the United States or overseas.

The FTU also serves as a liaison with national and international forensic laboratories. Examiners from this unit provide extensive training services, both general and specialized, to members of law enforcement agencies as well as to crime scene personnel and investigators. Training schools in trajectory analysis, gunshot residue analysis, firearms identification, and the identification of stolen motor vehicles are sponsored by the FTU on a yearly basis, often with multiple classes being offered in different cities throughout the year.

 serial number location  truck
The identification of stolen motor vehicles may involve trucks and construction  equipment as well as motorcycles, cars, and watercraft. FTU-sponsored training schools provide information on the location of serial numbers and how to determine whether they have been criminally altered.
 bullet trajectory training car
Bullet trajectory analysis provides information about the location or positioning of a shooter and the direction of fire. This photograph demonstrates how firearms examiners document the flight paths of individual bullets with regard to entrance and exit holes.


For more information about the Firearms-Toolmarks Unit, call or write:

Federal Bureau of Investigation
Firearms-Toolmarks Unit
Room 3787
935 Pennsylvania Avenue NW
Washington, DC 20535

Telephone: 202-324-4378
Facsimile: 202-324-8201

FTU seal