Firearm examiners at the BCA can evaluate items to determine the distance between the muzzle of a firearm and an object when it was shot. Although almost any object can be the target of gunfire, the BCA most frequently receives requests for proximity determination examinations on victim’s clothing.
When a firearm is discharged it emits smoke, soot and other residues, primarily from its muzzle. The residues exit the muzzle generally in a cone shape and can land on the target object surrounding the bullet hole. The size and density of the pattern of residues on the object can vary with distance, allowing a firearm examiner to determine the proximity of the shooter to the target when the gun was fired.
Examiners visually examine and chemically process evidence for the presence of gunshot residues. They use chemical tests that react with nitrites (a burned component of gunpowder) and lead (from contact with the bullet or the vaporous lead from the primer, or ignition source of a cartridge).
Examiners test fire the firearm used in the crime and same or similar ammunition at a variety of muzzle to target distances. This allows the examiner to reproduce the pattern of nitrites and the presence of lead residues. Comparison of the test targets to the evidence allows the examiner to determine a distance range.
Similarly, a firearm examiner can determine the approximate distance the muzzle of a shotgun was from a target by reproducing the size of the pellet pattern. Examiners test fire the shotgun at different distances and compare the pellet pattern to the pattern on the evidence.
The Minnesota Bureau of Criminal Apprehension Forensic Science Services does not currently conduct examinations on evidence or gunshot residue kits for the presence of primer gunshot residues. This examination involves the analysis of samples, typically collected from the hands of individuals, for the presence of primer gunshot residues with the intent of linking an individual to a shooting incident.
For information on independent testing labs offering primer gunshot residue testing services, please contact the Firearms Section.