Gunshot residue analysis is a common request in forensic laboratories with cases that question whether or not an
individual fired a firearm. When a firearm is fired, several types of residues are expelled from the openings on the firearm. Primer residues, specifically, are detected in GSR analysis. Primer residues are made of particles which contain elements found in the primer of the ammunition used in the firearm. The common Sinoxid-type primers can yield what have been characterized as ‘particles characteristic of gunshot residue particles.’ Like any combustion reaction, the chemical reaction responsible for firing a gun requires an initiator, fuel for the reaction, and an oxidizer. Sinoxid primers employ lead styphnate as the initiator, or primary explosive, barium nitrate as the oxidizer, and antimony sulfide as the fuel. When a gun is fired, the primer components vaporize due to the extremely high temperatures produced by the combustion reaction and are pushed out of the openings in the gun, due to the force of the reaction. Upon being released into the environment, the vaporized particles combine and resolidify into single particles, some of them containing a mixture of elemental lead, barium, and antimony. Along with the metals from the primer, gunshot residue particles may also contain reminiscent elements from the propellant mixture, metallic residues from the firearm itself, and residual elements from previous firings of the gun. Particles that are molten in appearance, due to formation in high temperatures, which contain lead, barium, and antimony, have been termed particles characteristic of gunshot residue, or ‘characteristics particles’.