The Mechanism of Gunshot Residue Deposition and Its Probing Characteristics to Reconstruct Shootings

Abstract

The significance of the predominant distribution of gunshot residue (GSR) spheroids, mainly on the back of the hand, has been explained by fundamental studies of the mechanism of GSR deposition and its probing characteristics to reconstruct shootings. These studies were based upon Glue-lift collection of GSR for examination in the scanning electron microscope with energy dispersive X-ray analysis. By performing “muzzle blast block” and “trigger-blast block” experiments with selected shotguns, it has been shown that in routine test firings of these weapons the breech GSR contribute the most to the GSR deposits on the shooter’s hands. Muzzle-blast GSR seldom settle from air, or by a blow-back, onto the shooter’s hands. The breech GSR are predominantly smooth, regular spheroids, whereas muzzle-blast GSR are mostly irregular and accompany bullet residues and bullet fragments. Since collections were made within a few seconds after each firing, the previous observation suggests that breech GSR are blown instantaneously onto the immediate surfaces of the shooting weapon, and on any hand in contact with, or in close proximity to the weapon. This forced deposition is an advantageous as well as cautioning phenomenon. If the gun is precleaned and the ammunition and the hand grasps remain unchanged, reproducible GSR counts can be obtained from single firings, providing that all deposited particles containing one, two or all three characteristic elements (Pb, Sb, Ba) of GSR are taken into account. This density distribution is a function of the weapon and the ammunition, and is therefore the basic criterion necessary for reconstruction of a shooting crime. Within each such distribution, the elements, sizes, and the morphologies of residue particles may vary significantly from shot to shot. Residue particles may also be transferred by contact from a freshly fired gun to a non-firing hand. In fact, more GSR may result in this way on the palm of a hand than on the shooter’s hand if specific areas of a weapon (e.g. cylinder and barrel of a revolver) are handled. If a shooting is attended or encountered at a very close distance (inches), similar GSR distribution may result on the shooter’s and non-shooter’s hands. However, if collections are made at the crime scene from undisturbed hands and the shooting incident is duplicated by test firing the suspect weapon with the ammunition in evidence, significant interpretation of the GSR collections may come out even in difficult cases. In such cases, the density distribution and the GSR characteristics are to be given strong consideration. The most important interpretation of all is that the positive indication of GSR, mainly on the back of the hand, implies a shooting hand only if it is a proven case of an unattended shooting.