When 3D printed (aka “additive machining”) comes to firearms parts that have to withstand pressure, they’ll work. There is already 3D machining of metal parts - the printing “ink” is powdered metal and binder, and the result is baked in an oven to sinter or fuse the metal together.
There’s already plenty of firearms parts that use something very much like this in MIM parts.
http://www.ar15.com/forums/t_3_118/579913_3D_printed_lower___yes__it_works_.html
Works just fine. Not quite up to printing a barrel yet... But getting there.