“3-D printing” is just a fancy way of making plastic objects using computer-controlled methods.
For making parts of guns, it might be possible to make the plastic parts this way. However, parts such as the chamber and barrel better be out of metal — and those parts can be made in a small, traditional machine shop. A lathe and a milling machine would do all that is necessary except perhaps the rifling, but the latter was accomplished long ago with simple hand tools.
A cross-bow is rather easy to make with ordinary hand tools.
In summary, all the talk of 3-D printing does not really add much to the story of guns. That might change if someone made a 3-D printer for metal objects, but then you are near to a computer-operated milling machine.
The only thing that stops more Americans from building guns is the law -- those capable are mostly law-abiding and those who are criminal mostly aren't capable, or interested as things are now.