Firing cycle dynamics and forensic interpretation of 3D ‐printed Ruger 10/22‐pattern .22 LR rifle receivers during live fire

Abstract

Polymer receivers fabricated by fused filament fabrication (FFF) are increasingly encountered in forensic casework; however, their firing‐cycle behavior and the stability of associated ballistic toolmarks remain poorly understood. In this study, Ruger 10/22‐pattern FFF‐printed receivers produced from polylactic acid (PLA) were evaluated under controlled indoor shooting range conditions alongside a factory‐produced aluminum receiver using identical trigger, bolt, and barrel components. Fired cartridge‐case first‐impact coordinates were recorded in three dimensions to quantify ejection dispersion and to estimate relative bolt kinetic energy, using the mean ejection distance as a conservative proxy. Under the improvised barrel‐retention configuration used in this study, the FFF‐printed receivers produced shorter and more variable fired cartridge‐case ejection ranges than the factory reference, corresponding to approximately 46%–57% of the factory receiver's relative bolt kinetic energy, depending on ammunition type and reference condition. This observation was consistent with greater sensitivity to assembly‐dependent factors, including headspace‐related variation and barrel retention instability, and with reduced bolt acceleration. To examine systematic changes in toolmark stability under conditions of reduced barrel‐to‐receiver rigidity relevant to improvised privately manufactured firearm (PMF) assembly practices documented in forensic submissions, an unsecured‐barrel configuration was used. Under this configuration, sequential firing produced progressive changes in ballistic toolmark expression: chamber‐contact regions diminished and shifted over short firing sequences, and firing‐pin impression footprints were consistently smaller and less distinct than those produced by the factory reference receiver. These findings provide practical insights for the forensic interpretation of fired cartridge‐case toolmark patterns, operability assessment, and evaluation of toolmark reliability in PMFs.