Why and when is firearms evidence examined?
Firearms evidence can usually be found at any crime scene where a weapon has been fired. These include crimes such as murders, armed robberies and aggravated assaults. According to crime statistics from the Federal Bureau of Investigation (FBI), in 2011 firearms were used in 68 percent of murders, 41 percent of robbery offenses and 21 percent of aggravated assaults in the US.
When evidence such as shotshell casings, cartridge cases, bullets and slugs is found at a crime scene—or recovered from victims, buildings, furniture, vehicles, trees, etc.—an examiner can analyze it to determine the type of firearm used. The examiner can also compare shotshell casings, cartridge cases and bullets from different crime scenes to determine if a common firearm was used.
When a firearm is recovered, the examiner can either identify or eliminate it as the firearm used in the crime, provided that (1) it still fires and (2) there is evidence such as a cartridge case or bullet for comparison. If the firearm is identified as the one used in the shooting, the examiner will measure the trigger pressure, determine if the weapon functions properly and ensure that the safety features are working, which can support or eliminate potential defenses such as accidental discharge.
In addition to obvious types of firearms evidence, the evidence left behind could include gunshot residue and powder burns. Examiners can use this evidence to determine distances involved in shootings so they can reconstruct the incident. This can be used to support or refute an account of the shooting.