I mean it depends, you could 3D print a lower receiver and fill in the rest with regular stock gun parts. The lower receiver basically holds ammo, and the trigger. In the case of a Glock for example it's the metal slide that does most of the work of actually shooting, the polymer frame is just there to hold everything in place.

This is why some guy was able to 3D print part of a Glock.

Europe is vague. In Norway it's definitely easier to buy one, than improvising this out of crap and sticks.