HOT FLASH! That genie has been out of the bottle for centuries. There's a series of Youtube videos showing a Williamsburg blacksmith/gun maker with his helper making and firing a flintlock Kentucky long rifle. And that was done with technology well over 200 years old.
When I was a child (6 decades ago) we would hear about inner city gang members making and using "zip guns" out of common hardware store available parts.
There is a scene in one of the Death Wish films where a good guy city dweller uses a shotgun made of plumbing parts.
I've seen videos of people making an AK-47 out of a shovel.
I've seen reports of the Khyber Pass gun makers making firearms with hand tools.
My own experience is of a fellow US sailor that I went through nuclear power school and prototype with that built a working (quite well I have to add) 1911. Thinking back I don't know if he made all the parts but I DO know that he machined the slide and frame (to JM Browning's specs) from steel stock using a Unimat bench lathe/mill. I'm not sure about the barrel but wouldn't bet against him having made that also.
https://en.wikipedia.org/wiki/Unimat
He also had several single shot handguns that he had made including boring and rifling the barrels that had nice, smooth working trigger/hammer mechanisms.
As I said and will continue to say, the genie's out of the bottle and ain't going back.
