The current study provides a quantitative examination of 634 firearms used in 348 mass shootings (1966–2018) through a unique firearm-level database. Specifically, this work identifies the relationship between the types of firearms, methods of obtainment, firearm regulations, and incident outcomes. Findings indicate the most common firearms were handguns. They were often legally obtained by the perpetrator, from independently owned or operated federally licensed firearms dealers. Although handgun-specific regulations did not appear to impact the legality of firearms, assault weapons bans were associated with an increase in illegal obtainment. Furthermore, the presence of a semiautomatic rifle and a higher number of guns were associated with increased casualties. A discussion of key findings provides important implications and future directions for scholars, practitioners, and policymakers.