Its not so much the machine that is doing the 3d printing it is the plastic being used, Like kanwar said there is alread 3d metal printers, the company I work for built one. Could you print a lower with it not likely but the new ones for sure. The legislation in place already says that you have to have the lower registered regardless of how you maunfacture it. In the states there is a law that says a firearm has to be able to be seen by a metal detector. The antigun groups are already changing there tune, they are going after the ammunition now, expect huge drives to limit then restrict ammunition and powder.
Defcad is cool, look up the site, the guy running the group has had consistant death threats.