Actually a lot of the components they're building the with these 3d machines are plastic based..Stuff like plastic molding lower recievers and magazines. It's actually pretty simple consumer based technology now. Companies like Nokio have already released schematics for people so they can basically download and input into the software and it'll do everything for em, just hit go... You can buy a small consumer one for $1200. They talked all about this early at the vegas tech expo..
Get a civil ar16 and modify and make a lower receiver and however big a mag you want and go to town.
“There are no restrictions on an individual manufacturing a firearm for personal use,†a Bureau of Alcohol, Tobacco, and Firearms (ATF) spokesperson said. “However, if the individual is engaged in business as a firearms manufacturer, that person must obtain a manufacturing license.â€