It can be also out of country purchases: go to Canada, buy the gun, come back. The situation is not different in any way, especially because if you buy the gun in Vermont or Canada you are still not allowed to get it in California. Do you want to regulate guns in Canada?

And that's assuming that people don't just manufacture the guns themselves - automatic rifles with box magazines are literally 1940s tech, and while making a good one takes a ton of expertise, something able to kill a bunch of unarmed civilians is a lot easier. For that matter, how long will it be before home CNC mills will be able to make modern(ish) weapons?

Don't ever bother with an automatic rifle, a semi-auto is much more precise and easier to build.

That horse has already left the barn.

There are custom made Cnc mills designed to make weapons that sell for less than a thousand bucks.

Soooort of, but not really - at least for the ones I've seen. There's a difference between "finish an 80% lower" and "make a gun from blocks of metal", and I think we'll hit the latter relatively soon.