A preliminary study of the evaluation of breech face impressions produced by various manufacturing methods

During cartridge case comparisons, firearm examiners must distinguish between different markings found on the cartridge cases. These characteristics can be classified into class, subclass, or individual characteristics. There is potential for a false identification if firearm examiners do not assess subclass characteristics carefully and mistake them as individual characteristics. Breech faces were manufactured by three different manufacturing methods (i.e., broach, plunge mill, and lathe) and two different finishing methods (i.e., glass bead blasting and tumbling). The manufacture resulted in subclass characteristics present on these breech faces. Ten test fires were collected from each breech face at each step in the manufacturing process. A confocal microscope was used to collect the 3D topographical scans, and pairwise comparisons were performed using the National Institute of Standards and Technology congruent matching cells (CMC) algorithm. The results revealed that carryover of subclass characteristics from the broached breech faces onto the breech face impressions occurred. The breech faces manufactured by plunge milling and lathe turning transferred minimal subclass characteristics to the cartridge cases. Using the ANOVA and Kruskal-Wallis H tests determined the presence of significant differences between all finishing groups except for the turned breech faces finished by glass bead blasting versus tumbling. In addition to the comparison of the cartridge cases using CMC, they were optically evaluated by comparison microscopy. The position of the CMC cells was indicated on these comparisons. The method of determining subclass characteristics by comparing the opposing edges of the breech face impressions on primers was also used.