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

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.