One of the most dependable techniques for proving whether a person has recently fired a gun is the analysis of gunshot residue (GSR) by scanning electron microscopy with energy dispersable X-ray analysis (SEM/EDX). 

     In the study of 17 GSR AND 19 reference samples, Jozef Lebiedzik et. al. used more than 500 rounds of commercially available ammunition and six different types of handguns. The individual particle X-ray composition was determined for 12 elements. The composition of GSR was analogous with barium oxide. "When present in a specimen, GSR could be sufficiently identified with automated procedures in less than an hour by restricting analyses to features bigger than two micrometers. In 'clean' samples, a higher resolution particle search was necessary to evade declaring false negatives. Careful control of the back scattered electron signal strength threshold, by reference to a standard, was needed to ensure both time-efficient and accurate analyses.

     Samples collected from non-shooting subjects, active in a physical environment which contained firearms discharge residue were seen to be easily contaminated by sub-micron GSR particles."