It's not "disproven" - when an organization can buy a $30k machine and crank out high-quality firearms all day long, you can't do anything to stop it.
All you have done is shown that you have no idea how difficult and time consuming machining is, vs. mass production.
So why aren’t they doing that today? Pretty simple empirical question.
The answer is that it is in fact easier to just buy them in the US.