A preliminary study of the manufacturing of breech faces

Firearm examiners may face challenges with subclass characteristics. Subclass characteristics have the potential to be confused with individual characteristics if a careful analysis for the presence of subclass characteristics is not performed. Several case studies are reported in the literature that discuss the presence of subclass characteristics and advise firearm examiners to disregard these characteristics for identification purposes. As far as can be determined, no fundamental study has been performed that explores the various methods used in firearm manufacturing and assess the potential of subclass characteristics being produced. In this study broaching, plunge milling, and lathe turning were used to produce consecutively manufactured breech faces for a Thompson/Center® (T/C®) Contender® G2 pistol. The breech faces were finished by tumbling or glass bead blasting. The 3D topographical scans of the breech faces were collected using a confocal microscope and then compared using the National Institute of Standards and Technology (NIST) congruent matching cells (CMC) algorithm. The results of this study demonstrated the presence of subclass characteristics after manufacturing by broaching and plunge milling. Lathe turning did not produce any subclass characteristics. ANOVA and Kruskal-Wallis H tests were used to determine significant differences between finishing methods within each manufacturing method. Significant differences were found in glass bead blasting versus tumbling and glass bead blasting versus no finish. No significant differences were found in no finish versus tumbling.