It just goes to show you that someone could clandestinely make darn near anything they wanted to with the right equipment, knowledge and time. Forging a decent barrel would be the hard part.
“Forging a decent barrel would be the hard part.”
A Sheriff friend of mine made all his own rifles mostly with a lathe that he shot national matches with.
He was an undercover narc and nothing short of deadly with handguns and rifles!!!
Fortunately, that piece is not controlled and requires no checks to purchase. With a blank of the proper size and a metal lathe, you can turn whatever you need.
Technology is indeed now at the point where you can make just about anything without requiring a large machine shop.
I would never advocate doing anything illegal, of course. Just affirming that home manufacture of darned near anything these days is indeed possible to do.