OP is probably talking about 3D printed devices. There are devices made of nothing but polymer (and maybe a spring? although I believe elastic can be substituted) that do function. They don't get many shots, but they will still put some projectiles downrange before malfunctioning. Some even have profiles which look nothing like a handgun, you can't rely on a human operator to recognize them when scanning bags.

Even more robust DIY guns like this use metal components for only the breech, and that's a very small component that could easily be confused for any number of other things.