Advancing forensic firearm examination protocol: Shotgun firing pin impressions

The theory of forensic firearm examination relies on the assumption that of the two contacting surfaces, the harder surface leaves marks on the softer one. The class and individual characteristics left by firearm parts undergo change due to several key variables, including the ammunition, and the firearm themselves. Notably, the characteristics are subject to distinct or permanent change during the first few fires immediately after manufacturing. In contrast, alterations arising from the many other factors associated with the shooting process tend to develop with slight variations. Using four different shotgun, this study examined the distinct changes observed in the individual characteristics formed by firing pins on fired shotshell cases. Understanding the manufacturing processes and conducting elemental analysis on the firing pins was necessary during this examination. The choice of material used to construct the shotgun firing pin appears to be the main reason for the inconsistent and changing marks observed on the fired shotshell cases. Although the changing individual characteristics within firing pin impressions could hinder the forensic comparison and examination of the shotgun, this study demonstrates how an integrated, holistic approach can overcome this challenge through the comparison and analysis of other firearm parts' marks. This study contributes to the scientific understanding of forensic firearm examination by evidencing the contribution and importance of the firearm part's material composition and the need to document the part's surface prior to test firing.