The Firearms Detail of the laboratory is responsible for the examination of firearms, cartridge cases, projectiles, and other firearms related evidence.
Types of Firearms Examinations
- Firearm Functionality
- Microscopic Comparison of Fired Bullets and Cartridge Cases
- General Rifling Characteristics (GRC) of fired bullets
- Ammunition Component Association (bunter mark comparison)
- Serial Number Restoration
- NIBIN Entry
- Gunshot Residue Testing
Firearm Functionality
Submitted firearms are examined to determine whether or not they function as designed. Trigger pull is measured, as are barrel and overall lengths and the functionality of safeties is determined. Firearms may be impact tested (tested to see if the firearm fires when dropped) when appropriate or requested. As a part of the functionality exam, the firearm may be test fired if safe to do so. Any departures from the normal firing/cycling process are noted. Test fired bullets and cartridge cases may be recovered and retained for subsequent examinations.
Microscopic Comparison of Fired Bullets and Cartridge Cases
Microscopic comparison of fired ammunition components enables the examiner to examine and compare marks left by a firearm on these components. Cartridge cases or bullets from shooting events can be microscopically compared to other shooting events or to known samples (test fires) from recovered firearms. These comparisons may allow an examiner to render an opinion as to if they came from a common source (fired in the same firearm) or a different source (not fired in the same firearm). Sometimes a conclusive opinion cannot be rendered due to firing conditions and/or damage (inconclusive- could not conclusively be identified or eliminated as having been fired in the same firearm). The examiner may be able to determine the number of guns fired at the crime scene or if a firearm has been used at multiple scenes (see NIBIN section).
A common instrument used in the Firearms Detail is a comparison microscope, which allows two items to be viewed side by side at the same time.
General Rifling Characteristics (GRC) of Fired Bullets
When no firearm has been recovered, examination of fired bullets may determine the possible make and caliber of the firearm which may have fired the bullets. The bullets are evaluated, weighed, and measured to determine caliber. The rifling on the bullet is examined to determine the number of lands and grooves present, the widths of these lands and grooves, as well as their direction of twist. Using this information, the AFTE (Association of Firearm and Tool Mark Examiners) General Rifling Characteristics (GRC) database is referenced and a list of possible firearms is generated.
Ammunition Component Association (Bunter Mark Comparison)
The bunter is the tool that is used to press the headstamp information into the cartridge case. This tool imparts toolmarks on the cartridge cases during the manufacturing process and these can be microscopically compared to make an association between cartridges and/or cartridge cases. Due to the fact a bunter may be used to produce up to hundreds of thousands of headstamps, this association is limited and only provides information regarding cartridge cases manufactured by a given bunter, which may span many boxes of ammunition.
Serial Number Restoration
Often, it is possible to restore a serial number that has been obliterated. Methods typically involve chemical etching and oblique lighting techniques.
NIBIN Entry
The National Integrated Ballistics Information Network (NIBIN) is supported by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). NIBIN is a computer database of digital images of fired cartridge cases. The marks imparted to a cartridge case by a firearm during firing are imaged and the images are stored in the database, which includes cartridge cases from crime scenes as well as test fired cartridge cases from seized firearms. The system searches previously entered images for similarity and produces a list of cases with similar images. The list is reviewed, and an association is made when images of two cartridge cases are similar. If a potential association is found, the evidence may be obtained for a microscopic comparison to confirm the association or “hit”. Since the system is most effective for firearms that leave cartridge cases at the scene, cartridge cases from revolvers are not typically entered into the system.
Gunshot Residue Testing (GSR)
When a gun is fired, particles of primer residue may be deposited on the shooter's hands, body, or clothes. These particles can be collected with pin-type carbon adhesive stubs and then analyzed with a SEM-EDS (Scanning Electron Microscope with Associated Energy Dispersive X-Ray Spectrometry). The presence of GSR indicates that the subject may have fired a gun, handled a gun or fired ammunition components, or was in proximity to where a gun was fired. Because GSR can be lost, negative results do not eliminate a subject has having fired a gun. The LVMPD Forensic Laboratory contracts this analysis to an outside laboratory.
Sample Limitations Policy
In order to ensure analysis of the most probative evidence, the Forensic Laboratory has implemented sample limitations for the analysis of GSR. For details regarding the sample limitation policy, please contact the Forensic Laboratory prior to submitting a Forensic Laboratory request that includes GSR analysis.