When ammunition is discharged in a firearm the primer, which contains a friction sensitive composition, provides a burst of flame which ignites the main charge of propellant, or gunpowder. The rapid burning of the propellant creates the high pressure which pushes the bullet from the cartridge case, and down the barrel of the gun.
Gases, vapours and particulates are formed during the discharge of the cartridge, and these are expelled from the barrel, along with the bullet, and from other openings in the firearm, such as the ejector port of a self-loading pistol. Referred to as gunshot residue (GSR) or firearm discharge residue (FDR), the particles are of organic and inorganic nature.
GSR will land on surfaces in the vicinity of the discharge, including the firer, and the recovery of GSR from a suspect may be used to suggest involvement with a shooting incident.
Residues produced by the primer are metallic, and will include different elements depending on the precise primer composition, providing a limited degree of discrimination between some types of ammunition. As a rule though it is not possible to identify the specific cartridge from which GSR originated, and it is quite possible for GSR to be transferred to a surface or person not at a shooting scene. Consequently it is important to have the nature of the contamination considered as well as the type, distribution and number of GSR particles when it becomes a factor in a criminal matter.