If one took parts fashioned on a 3-D printer and cast them in metal then machined them and assembled them, he could make a more traditional firearm but, I think that would defeat the purpose of making a quick, cheap gun at home. The only parts made on a 3-D printer that would stand up to the wear of use in a real firearm would be non-structural parts like stocks and handgrips, etc.
Even on an AR-15, the upper and lower receivers are made from cast or machined aluminum. In some places, only a few millimeters thick. All of the explosive force of the cartridge and bullet is contained by the barrel, chamber and locking bolt mechanism. The entire rest of the rifle could be made from plastic or boron/composite materials. Some versions of the AR-15 have been made from alternative materials but the chamber, barrel and bolt still have to be made from steel.
Consider that internal, chamber pressures inside a "civilian" spec. AR-15 can reach 50-60,000 PSI and go as high as 80,000 PSI in a military spec. M-16. (The "civilian" AR-15 and the "military" M-16 are similar weapons except the AR is not capable of fully-automatic fire where the M-16 is.)
Bottom line: I would not want to be anywhere near an all-plastic gun, made at home on a 3-D printer when it is fired! It would be just as dangerous, if not MORE dangerous to the person standing behind the gun than the one standing in front!
Let's stick to making cameras! I think it would be more fun, anyway.