You can only “print” the low stress parts. Barrels and bolts, firing pins and springs must still be metal.
There’s metal printing out there. Here’s one example:
http://www.exone.com/materialization/what-is-digital-part-materialization/metal
There are AR barrels that are being made from 420 stainless, as well as other gun parts. Machines about as well or a bit less smoothly than 416 stainless, which is the most common grade of stainless for stainless rifle barrels. For receivers and bolts, there’s some other grades used - 17-4PH is one.
Springs are a hot area of research in “additive” machining. The issue that interests me the most in this area is how they handle the heat treatment. Spring steel has to be hardened, then tempered, to make an effective spring. The additive machining MechE’s are looking at this from a different angle - creating a spring that’s very different than what we’re used to.
Another place to look: There’s a company out of Germany called “EOS” who is making laser sintering machines that can make for-real metal parts that can handle some real stress - think “gears” and such.
http://www.eos.info/en/products/systems-equipment/metal-laser-sintering-systems.html
I think we will see a for-real 3D printed gun - barrel, receiver, pins, springs, the whole thing - within 5 to 8 years from now. The economic winds that are howling through the manufacturing sector are making “additive machining” the next revolution - and it will be a bigger revolution than CNC machining was, IMO.
And not just any metal. High strength steel. Or a metal of equivalent strength.