The individual manufacture of guns has not been illegal or regulated at all until very recently, and then only ineffectively. California recently required people who wish to make guns at home to apply for a state-supplied serial number before they make the gun. The law has been largely ignored....
I have said that for some time... I am a fair machinist... Nothing special, but competently capable to handle, say, any machining necessary to rebuild an automotive engine, and experienced at same. I possess all the hand tools necessary to machining (dial indicators, micrometers, etc), part and parcel, as a normal part of any fairly well outfitted set of tools.
There is no magic here. A reasonably outfitted automotive/welding shop (which I have) with the addition of a simple tooling lathe and a fly cutter, and any reasonably adept mechanic can manufacture a gun from scratch...
The only part if it that would be a little baffling to me is cutting the rifling into the barrel. And that can be pretty easily overcome, I am sure.
It can be built pretty easily by any fairly competent and handy guy. That is not going to change, cannot be changed. How the heck can anyone control that?