Tracing the untraceable: A review and discussion on the forensic attribution of 3D-printed firearms

The rise of 3D-printed firearms (3DPFs) presents challenges for forensic science, law enforcement, and regulation. Unlike traditional weapons, 3DPFs can be manufactured using consumer-grade additive manufacturing systems and open-source designs, with the absence of serial numbers limiting traceability. Unconventional and inconsistent toolmarks produced during manufacture and firing complicate forensic attribution, or association, to specific printers and restrict linking bullets and cartridge cases to individual 3DPFs. Since 2021, prevalence has increased globally, with UK cases rising by 733% between 2021 and 2023. This work examines: (1) the expanding threat from 3DPF-related crime, (2) forensic challenges in linking 3DPFs to printers and ammunition, and (3) advances in quantitative identification. Conventional firearm examination techniques, particularly those relying on rifling impressions, are often unsuitable for fused deposition manufactured 3DPFs, which commonly use plastic or improvised smooth-bore barrels. Although firing marks can transfer to ammunition, their distinctiveness and reproducibility vary with design and manufacturing method, limiting reliability. Alternative approaches include analyzing nozzle drag marks, deposition striae, and base-plate marks for printer attribution, although variability in printers, parameters, materials, wear, and subclass characteristics may affect robustness. Spectroscopic and mass-spectrometric techniques show promise for detecting polymer and gunshot residues, including hybrid polymer–primer particles, but datasets remain limited. The UK's shift towards quantitative reporting has driven research into methods such as micro-CT, geometric measurement, and Congruent Matching Cells for conventional firearms; however, these remain underexplored for 3DPFs. Overall, attribution is feasible but complex, requiring continued interdisciplinary research and robust, 3DPF-specific forensic protocols.