A gun made by an individual, with the implication that it cannot be traced from the manufacturer.
This is another term invented by the media to discuss gun control. The concern is that a gun not made by a known manufacturer cannot be traced.
Federal law allows an unlicensed individual to manufacture a firearm. However, a firearm manufactured by an unlicensed individual cannot be transferred. It must be kept or destroyed.
While this has been true forever, the issue has received renewed attention due to the advent of 3D printing. The bar to making your own gun has been reduced from metalworking skill to 3D printing skill, and presumably anyone can do that. I suppose the day will come when someone commits a crime with a 3D printed gun, but as far as I know, that has never actually happened. So far, the concern over 3D printed guns is theoretical.
Similarly, there are “80% lowers” and frames, which require some basic metalwork to make them functional as guns. As far as I know, none of these have been used in the commission of a crime.
The reality is, acquiring an untraceable gun is, and has always been, relatively easy.