A study on the firing capabilities of 3-D printed gun part retrofitted onto the original manufactured firearm. “Ghost guns” refers to guns without a serial number, which actually only comprises a small portion of the true umbrella term, privately made firearms. Privately made firearms can be entirely 3-D printed, assembled from “buy build shoot” kits (can be purchased with a firearm already up to 80% assembled), or be composed of 3-D printed parts retrofitted onto an existing gun. A gun barrel is an unregulated gun part which does not have a traceable serial number but it is of forensic value because barrels are the part of the gun which impart rifling on the bullet which can be used by a crime laboratory to track down the gun and possibly the perpetrator. The appeal of 3-D printing an untraceable gun barrel which will not leave rifling marks on the bullet is that it is relatively easy to make and it can be disposable. In this study, the performance of the 3-D printed gun barrel was evaluated based on its ability to produce a lethal projectile when test fired into ballistic gelatin. A commercially available 3-D printer (Makerbot Replicator+) was used to print an exact replica of the SBS barrel (a side by side barrel) of a Derringer Cobray Model DD in 38 Special/357 Magnum modified to be chambered for a .25 ACP cartridge using PLA Tough filament (the strongest filament compatible with the original extruder of the 3-D printer). The results showed that the 3-D printed barrel did not have the strength to survive multiple test fires and that the bullet was not capable of penetrating ballistic gelatin deep enough to cause lethal damage.