First of all, most modern firearms barrels aren’t hardened. They’re not “dead soft,” either, but they’re nowhere near “hard.”
Most barrels I work on a lathe are in the 27 to 32 Rockwell “C” hardness scale. For 4140 steel (which is what they’re made out of), I’d call “hard” starting in the mid-40’s Rockwell “C” and up to the high 50’s from there. Stainless barrels in 416 stainless are in the low 30’s as well.
Second, the day is coming when 3D printing can do this. The precursor technology to this is MIM - metal injection molding. Those parts start out as a metallic powder + resin + binder goo that gets injected into molds about the consistency of cheeze-whiz... and then they’re compressed, heated until the metals sinter or fuse together, removed from the molds, then they can be hardened as normal steels. They have 98% of the density of “real” steel when they’re all done. I’ve polished, ground, and worked on MIM parts in S&W revolvers that, except for their surface finish when I start, I can’t tell the difference in how it feels under my fingers from regular steel.
The powdered metals are the future of production metallurgy. “Additive” machining is also the future - product COGS will go down because instead of starting with a big chunk of metal and carving everything away you don’t need, you will use only as much material as you need to build it up. The push from industry to head in this direction is ferocious. IMO, the CNC machine companies better be looking over their shoulders...
The future 3D printed barrel might not be sintered while it prints, but a printed barrel is coming. Have no doubt about that.
The 3D printer was used to make the lower reciever on an AR. In the United States, this is the part of the AR that is legally controlled. All the other parts can be bought freely without government permission.
It is also legal for you to make your own lower reciever without government permission. Quite a few are already doing it. This technology simply makes it easier.
This shows some 3D printing in impressive metals:
Additive Manufacturing using metals
http://3dprintingsystems.com/additive-manufacturing-using-metals/