So, 3D printers

[AtheistCrusader]

8. You can't print out a working model.

Several parts in the gun must be made out of metal, and not just any metal, but forged, heat treated metal. Other parts also need to be metal, not to withstand pressure, but to withstand elastic strain and shock.

There are parts you can make out of plastic or printed metal, but only some of them. For instance, an AR lower. That can be done today. The quality isn't great, but it's there. But the parts inside are still going to be metal. And that's only about 1/3 of the gun right there. The upper needs to be a lot stronger to handle the strain of the bolt slamming back and forth, to say nothing of the bolt itself, the chamber and barrel, etc.