Confocal microscopy analysis of breech face marks on fired cartridge cases from 10 consecutively manufactured pistol slides

Recent publications from the National Academy of Sciences have called for additional foundational research in the field of firearm and toolmark analysis. We examined test fires from 10 pistol slides with consecutively manufactured breech faces. A total of nine test fires from each pistol slide, for a total of 90 test fired cartridge cases, were compared using confocal microscopy combined with three-dimensional cross-correlation analysis algorithms. A total of 8010 comparisons were performed (720 matches and 7290 nonmatches). The average score for matches was 0.82 with a standard deviation of 0.06. The average score for nonmatches was 0.20 with a standard deviation of 0.03. Additionally, subclass toolmarks were observed on the breech faces, but the presence of subclass was not detected in the correlation analysis. There was no overlap of scores between matching and nonmatching test fires. This provides objective data that support the AFTE (Association of Firearms and Tool Mark Examiners) theory of identification.