How to recognise the traces left on a crime scene by a 3D-printed Liberator? Part 2. Elements of ammunition, marks on the weapons and polymer fragments.

The Liberator is a firearm that can be manufactured from its blueprints by the means of a 3D-printer. This handgun is composed of nineteen pieces: eighteen made of printed polymer and one metallic nail. This study focuses on the physical traces which can be found and exploited from a forensic point of view after the discharging of such types of pistols. Two main aspects have been investigated: (1) whether it can be inferred that a 3D-printed handgun was used when investigating a scene; (2) when presuming the use of a 3D-printed Liberator on a scene. Six Liberators were manufactured, assembled and discharged. The discharge occurred under controlled conditions to allow the collection of ballistics data and traces produced by the shooting. Elements of ammunition - cartridge cases and projectiles - that were fired during an experimental campaign appeared to carry polymeric materials (flakes, melted polymer or both). Besides, we observed that the fired ammunition elements lacked traditional marks left by the metal pieces of a conventional firearm. Indeed, the projectiles did not carry rifling marks and the cartridge cases were found torn or swollen. Fragments or larger pieces of polymer were found, mostly on the ground, near the location of the discharge, up to nine metres away. The impression of the cartridge case head stamp was also found on a part of the Liberator called the "hammer body". This study showed that the discharge of a 3D-printed Liberator is expected to produce traces that can be transferred onto and recovered from the printed firearm, the cartridge case, the projectile, the target, and the environment of the discharge. These traces are different from those left by conventional firearms. When found on investigation scenes, some of these traces can inform on the use of a 3D-printed handgun and contribute to the reconstruction of a shooting event involving such a weapon. This study suggests that the approach adopted when investigating a scene must be adapted in terms of traceology when the use of such firearm is suspected