Gunshot Residue (GSR)
When a firearm is discharged, the primer compound detonates, igniting the propellant powder and expelling the bullet. This process produces a cloud of gunshot residue—microscopic particles containing characteristic elements from the primer (lead, barium, and antimony in traditional primers; strontium, potassium, and other elements in lead-free primers). GSR particles deposit on the shooter's hands, face, and clothing, and on surfaces near the muzzle. Collection uses SEM stubs—adhesive-coated aluminum discs pressed against skin or clothing surfaces. Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) identifies GSR particles by their spherical morphology (formed from molten metal cooling in flight) and elemental composition. Finding a particle containing lead, barium, and antimony simultaneously is considered characteristic of GSR.