I'm not sure what the big deal is. Individuals have been smithing guns since their invention. Hillbillies in the Ozarks could make rifled barrels. People with a manual or cnc mill in their garage are making ar15 lower receivers from billet aluminum. The 3d printing technique takes a lot of the skill out of it, but at the moment all you are getting is a low quality plastic gun. As a practical matter I don't think the swapping of gun part cad files is a significant vector for the creation of guns that are used to do bad things.