Scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX) was used to determine new structural criteria to further substantiate the view that gunshot residue (GSR) particles have a characteristic structure. Because GSR particles are formed by rapid cooling from extreme temperatures and high pressures, they should contain features of condensates not only on their surfaces, but also within their interiors. Both the surfaces and the cross sections of GSR were examined for X-ray mapping of elements and for topographic analysis. Vaporized lead, antimony, and barium may condense uniformly and concurrently, or irregularly and discontinuously, or as a layer of lead around a nucleus of barium and antimony. These three modes of GSR formation may correspond to the equilibrium state, the increasing temperature state, and the decreasing temperature state of the explosion gas mixture of the priming compound. Most GSR smaller than 10 $#X3BCm are formed as droplets at equilibrium. Larger spheres grow by coalescence of the smaller droplets. These residues pass through various metastable forms and then freeze. Only a few semisolid spheroids of barium and antimony may capture lead vapors of the etched bullet and burnt residues, and these appear as “peeled oranges.”

The mechanism of gunshot residue (GSR) deposition and its probing characteristics have been studied by the Glue-Lift collection of GSR and its identification by scanning electron microscopy and energy-dispersive X-ray analysis (SEM-EDX). By blocking alternately muzzle-blast residues and trigger-blast residues of shotguns, it has been shown that, if the firearm is thoroughly cleaned before the firing, the muzzle-blast residues seldom settle from the air onto the shooter's hands. Whereas the trigger-blast residues are literally blasted onto the immediate surfaces of the firearm and on both of the shooter's hands that are on the weapon. Therefore, the hand deposits are mainly the breech deposits. If the firearm is not pre-cleaned the residues of previous firings lodged as fouling in the interior of the gun are blown off rather irregularly through the breeches and the muzzle in the subsequent firings. These residues occasionally overlap with the fresh breech deposits of the shooter's hands. Color tests for residue developed with sodium rhodizonate have confirmed the basic mechanism of GSR escape through the breeches and the ejection mechanism of the close-breech weapons. The forced deposition of the trigger-blast residues is an advantageous as well as limiting process. The contrast of residue deposits on the back of a hand versus the palm, is due to shadowing of the residue particles by the hand grasp on the firearm. But these particles may also be transferred to the nonfiring hand by contact with a fired gun. Whether it is a handgun or a longarm, if the gun is pre-cleaned and the ammunition and the hand grasps remain unchanged, a fixed amount of residues is deposited per firing on the back of the trigger hand. This deposition, which takes into account all deposited particles containing one, two, and three characteristic elements of GSR (e.g., Pb, Sb, Ba), is a fundamental piece of information helpful for the reconstruction of a shooting.

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