When a firearm discharges, it doesn't only send a bullet forward. It also expels a fine cloud of particles backward and sideways — gunshot residue (GSR) — that settles on the shooter's hands, clothing, and nearby surfaces. GSR is useful evidence, but it is routinely over-interpreted, so it pays to know exactly what it does and doesn't show.
What GSR is
GSR is made up of microscopic particles from the primer and propellant, characteristically containing lead, barium and antimony fused together. Their specific composition and morphology are what let an analyst distinguish true GSR from everyday environmental particles.
How it's detected
Samples are collected with adhesive stubs from hands and clothing, then examined by scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX), which identifies individual particles by shape and elemental make-up. It's a particle-by-particle confirmation, not a colour test.
What it can and can't prove
Can suggest: that a person discharged a firearm, was near one when it fired, or handled a fired weapon or surface.
Cannot prove: who pulled the trigger. And several factors weaken the inference:
- Transfer and contamination — GSR moves easily; it can be picked up from a police vehicle, handcuffs, or another person, so it can appear on someone who never fired a gun.
- Time — it is loosely held and is lost within hours through normal activity or washing, so a negative result proves little.
- Environment — activity and setting affect how much is present.
The bottom line
GSR is corroborative, not conclusive. A positive result places a person in the vicinity of a discharge; it takes the whole picture — witness accounts, ballistics, scene evidence — to say what that means. Treating GSR as proof of "the shooter" is exactly the kind of overreach a good court will challenge.