When a gun is fired small particles of firearm discharge residues are produced from the bullet and cartridge case of the ammunition. Firearm discharge residues escape through openings in the gun e.g. the muzzle and breech, and can be deposited on the shooter and nearby surfaces.
Physical contact with a source, such as a gun, can result in the transference of discharge residues.
When a cartridge is discharged, small particles of Firearm Discharge Residue (FDR) are produced from the cartridge case components of the ammunition.
FDR can be recovered from the hands, face and hair of a suspect, from their clothing and from surfaces, for example from within a vehicle used to transport the suspects, weapons and cartridges after a cartridge has been fired.
The length of time that residues persist on surfaces is dependent on activity; residues can persist indefinitely on undisturbed surfaces and in enclosed spaces, conversely they can be lost quickly from hands.
The forensic scientist will examine the FDR to determine if samples from the recovered evidence match those of the suspect(s).
Due to the nature of the evidence strict contamination procedures and quality control checks are in place in the laboratory to ensure that no residue remains after each stage of the analysis.