Lawyers, judges, and juries can be seriously misled by crime laboratory findings of the presence of gunshot residues (GSR).
Gunshot residues are defined in the Association of Firearms and Toolmark Examiners (AFTE) Glossary1 as “ The total residues resulting from the discharge of a firearm, it includes both gunpowder and primer residues, plus metallic residues from projectiles, fouling, etc.”
To most AFTE members, this definition is reasonably understood; however, gunshot residues are often confused by members of the Criminal Justice System. The metallic cartridge (one round of ammunition) that is loaded into the chamber of a firearm consists of a bullet (projectile), gunpowder (propellant), a primer (igniter), and a cartridge case which contains all of the components before firing. When the primer is struck by the firing pin, a flash of hot gases ignites the gunpowder causing it to deflagrate (“explode”). The bullet is propelled from the cartridge case and forced through the gun barrel by the expanding gases.
Attorneys and judges often confuse gunshot residue examinations of primer particles containing lead (Pb), barium (Ba), and antimony (Sb) with examinations of the remaining elements of gunshot residue. Although the examinations of primer particles and those of gunpowder residues are examinations of gunshot residues, they are examined in very different ways. Primer particle examinations utilize instrumental techniques while gunpowder residues utilize visual and chemical methods.
The examination of gunshot residue primer particles is currently a controversial issue. Beginning with the guidelines given by the Aerospace Report of 1977,2 various GSR examiners have developed techniques, methods, and most importantly, opinions concerning the significance of their findings. There is a wide range of opinions offered in cases around the country.
As GSR examiners have advanced over the past forty years, there is little to suggest they have advanced in concert. The standards for expressing an opinion in one crime laboratory may be very quantitatively different than the standards in another laboratory. Fundamentally, GSR examiners are seeking microscopic particles containing lead (Pb), barium (Ba), and antimony (Sb) which, for the purposes of this article, are found uniquely in combination only in the primers of firearms cartridges. A particle composed of PbBaSb, fused together in a single unit, which exhibits the correct morphology (shape and appearance) to an experienced examiner is termed a unique particle. The examination itself consists of computer-controlled scanning electron microscope examinations of samples from suspect areas (commonlysuspect shooter’s hands) to locate the particles coupled with an X-rayanalyzer and an EDS, for energy dispersive analysis of X-rays to determine the elemental composition of the particles. In their book Current Methods in Forensic Gunshot Residue Analysis, A.J. Schwoeble and David L. Exline3 offer a thoroughexplanation of the process.