For hundreds of years people have been making guns without 3D printers and CNC mills. All that is needed is some metal machining skills, a lathe, and some other tools.

I have a copy of https://archive.org/details/gerard-metral-gun/ on my bookshelf (by a European, too!).

If you want to be really simple you can make a slam fire shotgun with two pipes and a nail, no machining skills required