There's also just plain hand craftsmanship, which is how they make modern military weapons in Darra Adam Khel, Pakistan. But that takes considerably more training and experience than a 3D printer. The Reason, Ghost Gunner, and 80 percent AR frames also depend on the fact that only the firearm receiver is regulated by federal law, allowing you to get all the other parts of the gun by mail order.
The big problem, though, is ammunition. Black powder is pretty easy to make yourself, but modern smokeless powders require considerably more sophisticated chemistry. Lead bullets are easy to make yourself, and there are plenty of molds available. But modern copper-cased bullets are more difficult. Extruding brass cases is also more than most of us are up to. And primers are the hardest.
Still, it's good to see home-made guns getting easier to make all the time.