Firearms produce a range of residues upon discharge, including chemicals from the primer, explosive, oxidizers, reducing agents, sensitizers, fuels, and binders (Romolo & Margot, 2001). Collectively, these materials are often referred to as gunshot residue (GSR). In current forensic practice, GSR refers specifically to particulate residues that are formed from compounds found in the primer, including metal oxides. The term "primer discharge residue" is sometimes used to make this distinction clear. The propellant is ignited by the primer. The propellant burns and generates the gas that forces the bullet out; it is the source of the organic gunshot residue (OGSR). Both GSR and OGSR are deposited on the hands of the shooter. Particles can also be transferred to clothing or others nearby—a process referred to as secondary transfer. Because of the possibility of secondary transfer and environmental interferences, it can be challenging to interpret analytical results, be they positive or negative. The analysis of OGSR could add to the evidentiary value of GSR evidence by providing additional information from a source of evidence not prone to secondary transfer.

Current laboratory-based forensic analytical methods that target residue from firearms discharge focus almost exclusively on primer residues. Typical primer chemical composition includes lead styphnate (initiator), antimony sulfide (fuel), and barium nitrate (oxidizer), which combine in a violent chemical reaction that ignites the gunpowder in a cartridge. The primer materials vaporize and re-condense to form tiny particulates that contain lead, antimony, and barium, and these particulates can be collected using metal stubs coated with carbon tape.

The presence of GSR on a stub is determined using a combination of scanning electron microscopy (SEM) coupled to x-ray spectroscopy, usually energy dispersive x-ray spectroscopy (EDS). The SEM is used to locate particulates that are within the right size range and that have a smooth, rounded morphology. The chemical composition is evaluated using EDS, which identifies particles containing the three metals of interest. This method is described in the American Society for Testing and Materials (ASTM) E1588 Standard Guide for Gunshot Residue Analysis by Scanning Electron Microscopy/ Energy Dispersive X-ray Spectrometry. The combination of SEM/EDS for characterization of GSR has been accepted for many years in forensic science.

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