It's an interesting idea, but pretty much impractical. You're not going to get anything that is anywhere nearly as durable as today's quality firearms (note, I did not include pot-metal zinc alloy POS guns...). You still need metal parts in a firearm, such as reinforcements, springs, ejectors, barrels, slides, etc. By the time someone has gone through the machinations to get a 3D printer working to produce such weaponry, he could have already purchased several standard firearms or even crafted his own at a much cheaper price.
Milling machines produce parts that are far more durable, and cheaper. Furthermore, they don't require nearly as much of a capital investment, and can operate under less than ideal conditions. After all, the gallant British folks were churning out STEN submachine guns in decrepit factories that had been bombed several times, during WW II.
Computer-controlled milling machines make it even easier, and are quite plentiful.
