Yeah, the one all plastic gun i saw, “the liberator” which was one if not the first ones made, is huge and bulky, and has what are effectively disposable barrels, since they are either gonna blow up on the first shot, or the second.
A more reasonable option would be a hybrid, where you use actual metal parts, but use a printer to make a housing assembly for them… But at that point you are effectively just making cosmetic crap.