This raises a question in my mind. A polymer gun, when fired, would quickly disassemble itself and would likely seriously injure or kill the person who fired it. My guess is that the barrel and components that make up the firing chamber were forged steel. I worked for a major space oriented company in the eighties and nineties and, toward the end of my tour of duty, we acquired a "printer" that could replicate mechanical parts. Those parts "were not" usable in any mechanical device that we created though. They were simply used to verify proof of design. I can assure you that, should you replicate a gun in one of these new "printers" and attempted to load and fire it, you would require a trip to the hospital or morgue.
Use the 3d printer to make your castings around. And then use high strength polymers, or metal.
Just an idea
The AR-15 design is unique; you can buy all the metal parts without dealing with a federal firearms licensee. The lower receiver is the serial-numbered part, but it is subject only to minor stress due to recoil. Some companies already make polymer versions of that part (with small metal serial-number plates imbedded), and the polymer versions are thicker than the mil-spec aluminum original.
So you can "print" copies of the only part that BATFE cares about in this case. Of course, you can also buy an incomplete (un-serialized) aluminum lower receiver and easily complete the machine work, but for some reason a $400 Harbor Freight milling machine isn't considered as big a game-changer as a 3-D plastic printer. Go figure.
“I can assure you that, should you replicate .... load and fire it, you would require a trip to the hospital or morgue.”
Sintering metal powder with a laser could produce very strong material with a 3-d printer. It would be a specialized printer, but it is doable.
Won't last. 3D printing in metals is available today (though quite expensive), yielding products that are as strong or stronger than those machined "normally" (better control of grain size, and spot-customized heat treatment throughout the part, etc). Quite soon, guns and other devices made by 3D printing will perform better than standard machined.
“I can assure you that, should you replicate a gun in one of these new “printers” and attempted to load and fire it, you would require a trip to the hospital or morgue. “
You’re being too simplistic and not following the 3D printer technology. Lower receivers and magazines can be seen operating on YouTube. If you don’t want to believe it, get left behind in the dust.
There are millions of freelancers perfecting 3D technology.