@magic8ball is right, the plastic 3d printed guns are really just for demonstration and not for practical use, they are just to prove that a functional gun can be made, just because they function for legal and demonstration purposes that is all they are intended for. If you want to "print" a "3d gun" for use then your best bet right now is a tabletop CNC machine that Defense Distributed, the same company that invented the plastic guns, came up with specifically for finishing metal lower receivers.

or you could use a 2d printer to print out a paper templated that you glue to a piece of bar stock and then mill out with a drill press and make your own AK47, as long as it is legal for you to own a gun it is legal to make that same gun.