Testing Trigger Pull
Bullet and Cartridge Case Comparisons
This microscopic examination enables the examiner to find unique marks left by a particular firearm on the cartridge cases and/or bullets. A careful microscopic inspection of cartridge cases or bullets from a scene may be used to identify or eliminate a suspected firearm as having been used in a crime. When no firearm is recovered, 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).
Caliber/Gun Make Determination/General Rifling Characteristics (GRC)
When no suspect firearm has been recovered, this examination determines the type and caliber of the firearm which may have been used to shoot the bullets recovered from crime scenes and/or victims. The bullets are weighed and measured to determine caliber. The number of lands and grooves visible in the rifling impressions and their direction of twist are determined. The land and groove widths are measured. The FBI’s General Rifling Characteristics (GRC) database is referenced and a list of potential firearms is generated. Fired cartridge cases often aid in this examination and should be submitted with the bullets if available.
Ammunition Component Association
This examination normally involves associating two different sources of unfired ammunition together or associating fired components (bullets and/or cartridge cases) with unfired ammunition. The tools that are used in the manufacturing process to press the headstamp information and cut the extractor groove leaves toolmarks on the cartridge cases and these can be compared to make an association between two sources of cartridges or cartridge cases. The bunter and extractor marks are compared to see if the cartridge cases were manufactured on the same machines near the same time. When fired bullets are submitted with unfired cartridges, the design and elemental composition can be compared to determine if they are consistent with originating from the same manufacturer.
Toolmark examinations usually involve comparisons of marks left at a crime scene by a suspected tool. Marks may be produced from a variety of tools including cutting, prying or gripping tools. Marks on large objects that cannot be collected or transported should be cast, using a casting material such as Mikrosil™ or Forensic Sil™. In addition, a toolmark or toolmark cast can be submitted in an attempt to provide the investigator with a possible tool type and or size of tool that may have produced the toolmark.
Serial Number Restoration
In many cases, it is possible to restore and determine a stamped serial number that has been obliterated. Methods typically involve chemical etching techniques.
Pre-Restoration Post Restoration
Distance Determination / Proximity Testing
In circumstances where a shot was fired in relatively close proximity to the gunshot victim or to a surface, gunpowder or soot patterns may be found. By comparing these patterns to standard patterns generated in the laboratory, it may be possible to give an estimation or range of distances as to how far away the muzzle of the firearm was from the victim. The general procedure for this type of examination involves test firing from various distances into similar type materials as those recovered from the victim or the scene. Then these patterns are directly compared to the evidence and subsequently the evidence and test patterns are enhanced using chemical reagents that react to the presence of lead and/or nitrites. Distance determinations may also be performed by examination of shotgun pellet patterns. Shotgun patterns are directly compared to test patterns by comparing the “spread” of the pellets.
Accurate distance determinations will generally require submission of the firearm in question and ammunition that is the same or similar to that used in the crime.
The National Integrated Ballistics Information Network (NIBIN) is supported and managed by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATFE). NIBIN is a computer database consisting of firearm related digital images. These images include certain microscopic marks produced on a cartridge case when fired. The database includes cartridge cases from crime scenes as well as test fired cartridge cases from seized firearms. Since the system is most effective for firearms having the potential to leave cartridge cases at the scene, cartridge cases from revolvers are not entered into the system. When a cartridge case is entered into the system, the computer searches the database for images that most closely match the cartridge case just entered. The examiner then visually compares the displayed images and looks for an association. A “potential association” is indicated when an examiner finds images of two cartridge cases that may be identified to a common firearm. If a potential association is found, the actual evidence must be obtained so that a traditional microscope comparison can be performed to confirm the association or “hit”.
Gunshot Residue Testing (GSR)
When a gun is fired, particles of expended gunpowder and primer residues often blow back onto the shooter’s hands, body or clothes. These particles are collected with adhesive lifts and then analyzed using a technique known as SEM-EDS (Scanning Electron Microscope with Associated Energy Dispersive X-Ray Spectrometry). The LVMPD Forensic Laboratory contracts this analysis to an outside Laboratory.
Sample Limitations Policy
In order to ensure analysis of the most probative evidence, to meet casework demands and to provide for responsible use of resources, the Forensic Laboratory has implemented sample limitations in the Firearms/Toolmarks unit in regards to 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.