Hmm.... need a CNC. Really?
Not at all. Guns from early on were made without the benefit of milling machines as we know them. The old Foxfire series of books had one dedicated to making black powder and a muzzle-loader. The thing even had rifling for accuracy. Maybe not a mass killer but it works.
Once you have a good grade of steel, you can build them like they did prior to WWI, by hand using power tools only for the really tough stuff like boring out the bore and chamber.
Why fiddle with an M-16, the old M-14 just needs a selector installed to control the sear.
As for good guys vs. bad guys, who has guns, who doesn't, etc. I'll leave that to everyone else to figure out. But it doesn't take high tech tooling to make something like a gun. Good grief, I made a working scale model of a British 9-pdr, circa 1790, that I made from brass. Yes, it could be scaled up and rather easily.