When a firearm is shot, in addition to the projectile(s), a mass of debris comes out the muzzle. These gunshot residues (GSR) can include various primer residues, residues from projectiles, and partially burned and unburned gun powered particles. The examination and analysis of GSR on items of evidence can allow determinations to be made as to weather a hole or defect is consistent with being caused by a bullet (or other firearm-related projectiles). Once in the Laboratory, the Firearms Section can examine patterns of GSR on items of evidence to determine muzzle-to-target distance.
Additionally, GSR can be transferred to an individual by discharging a firearm, handling a firearm or fired ammunition components, or by contact with another object that has GSR on it. The presence of GSR on a person may provide useful information linking an individual with an action that could transfer this residue to them. As a very general guide, after four to eight hours it is unlikely that residues will be found on a live and mobile individual's hands unless steps have been taken to preserve such evidence (e.g. bagging the hands). The residue can persist for longer periods of time on some areas of interest such as on the deceased, on clothing or other stationary objects. The decision to collect a sample is affected by many variables and must be based on the investigative information available. This type of analysis is not performed by the Laboratory, but the crime scene team will collect samples for analysis by an outside laboratory. The Laboratory can provide a list of outside laboratories that will do this examination for a fee.
The image below shows GSR coming from the muzzle and cylinder gap of a revolver that has just been fired.