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.