<< What’s the barrel and receiver made of? >>
That’s what I want to know. You need steel for that and you can’t do it with a copy machine.
Some in Congress have their panties in a knot and want to renew the ban, that I believe is due to expire, on “invisible” (to X-Rays) guns that was passed when Glocks started becoming popular.
The geniuses in Congress figured that if the Glock frame was made from polymers the pound of steel that made up the slide and barrel would become invisible to X-Rays and could get past the airport screeners. I think Schumer was behind that one.
<< What’s the barrel and receiver made of? >>
To the BATFE, a “firearm” is the piece that gets the serial number. So you only need to make the lower receiver. All the other parts (upper receiver, barrel, trigger group, springs) can be bought over the counter with no paperwork. (Although if “printing” lower receivers becomes popular, expect some legislation changing this).
The barrels and upper receivers can be purchased without any restrictions whatsoever — only the lower receiver is the ‘firearm’.