Divergent gunshot residues and characterization of the memory effect in a.22 caliber revolver and pistol

Abstract

Background: The forms and compositions of primer gunshot residue (GSR) in shooting cases have been routinely analyzed in many crime labs by scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS). Gunshot residue studies over the last 30 years have identified a contribution by residual GSR from surfaces within the firearm to GSR (the “memory effect”). In this study, we first focus on the GSR compositional differences between target, bore, and cylinder gap for a.22 caliber revolver which has received little attention in the literature. Secondly, we examine the behavior of elements involved in the residual contribution to GSR for a.22 caliber pistol. Results: The compositions of target, bore, and cylinder gap GSRs from a.22 caliber revolver were determined for 11 discharge series. Breech GSR compositions from a.22 caliber pistol changed in sequential discharges revealing behavior of individual primer and bullet elements. The target GSR forms from some.22 caliber ammunitions were not spherical. Conclusions: For the revolver, GSR compositions from the target, bore, and cylinder gap are divergent. Analysis of the pistol’s breech GSRs indicated mixing with the previously deposited GSRs resulting in a sequential contribution or loss of elements and redeposition of the new composition. Aluminum did not contribute to the residual GSR. The GSR composition of a discharge and its contribution to residual GSR on the internal parts of the firearm represents a complex process which likely involves a number of variables including, but not restricted to firearm design, primer composition, pressure and temperature of the discharge gas, and bullet surface composition.