This then brings up the important question, how do you "...Restrict the use of a a 3D printer to make guns.."? Is it at the hardware level, with some analysis engine used to determine if the shape matches some predetermined "firearm" pattern? Is it in the 3D CAD software that flags weapon like designs? or does it get so bad that the purchase of any 3D printer (CNC machine, mill, lathe, drill press, sander, dremel, will also eventually fall into this category) will require the purchaser to jump through hoops similar to steps needed to purchase class 3 firearms? Afterward, the operator will have to be licensed to operate the machine and personally verify the design being generated.
