Are gun barrels unregulated in most US states? E.g. in Slovakia a barrel is considered a main weapon part, and case of short arms the slide is considered another main weapon part. So a Glock has serial numbers on the barrel, slide and also the body. You can buy the rest without any license or manufacture it if you want, but no one will sell you a barrel or slide unless you have a purchase permit. You can't sell it to someone else, you need to transfer the ownership. And making a good and safe barrel and slide is the most complicated part, you can't just easily print this at home. So no one is printing guns in Slovakia yet (AFAIK :-)). And just for comparison - we have less strict rules than California (concealed carry is possible for regular citizen. Not limits on calibers or magazine capacity yet).