The more interesting part of this development is the possibility to make receivers on your printer. For example, the only federally controlled part on an AR-15 is the lower receiver. Every other part can be bought with no paper work (e.g., barrels, triggers, upper receivers, stocks, optics,...). There are already production models that use polymers. Factories that do this type of stuff require an FFL (federal firearms license) for manufacture of weapons. If you can do it in your house all the rules are out the door (legally you are also supposed to have the FFL, but...). You can make the receivers and buy the rest of the parts with cash for a fully untraceable gun. Another interesting point is that there are only very small difference between fully automatic versions and semi-automatic versions of the AR-15, if you can make the receiver at your house you could make a full auto version.