As forensic evidence goes, gunshot residue can be full of holes. But a new technique could provide a more definitive link between suspect and gun. A tool that employs lasers may allow investigators to match residue alone to a specific caliber of firearm, scientists report in two recent papers in Analytical Chemistry.
“Anything that’s going to enhance or expedite the detection of gunshot residue and provide stronger evidentiary value is a way forward,” says analytical chemist Jason Birkett of Liverpool John Moores University in England. “This work is very good and very novel and will do nothing but assist.”
Along with the bullet, a cartridge or round also contains a propellant (usually gunpowder) and a primer. When the firing pin strikes the primer cap, the primer ignites, igniting the propellant and expelling the bullet from the gun. “It’s a very complicated chemical reaction involving extremely high temperatures and pressures,” says Birkett. This reaction results in a spray of residue that can land on clothing, skin or anything nearby.
The resulting gunshot residue includes burnt and unburnt powder, primer, smoke from the combustion and grease and metals. Gunshot residue analyses typically use scanning electron microscopy to assess particles of barium and antimony (from the primer) and lead (from primer and bullet). Finding these particles indicates that a gun was fired and can help determine the distance the bullet traveled, but the approach is time-consuming and often not specific enough to link the residue to a gun of a particular caliber. These particles can also come from other sources, such as fireworks or the bits of dust some machine-shop workers might encounter. And to further complicate things, lead and other metals are being phased out of some cartridges for environmental reasons.