And this right here is why the "Ghost Gun" thing is largely just a scare like the Halloween drug thing that happens every year. Because it's generally cheaper and easier to go one state over to the state with lax gun laws and buy a gun there for the price of an Ender 3 instead of going through all the effort of buying a printer and learning how to use it. Same reason why the Mexican cartels smuggle guns out of the US and into Mexico, and not the other way around. Guns are cheap and plentiful in the US, and they're not hard to get.

There's some "teh gubernment is cummin fer muh gunz!1!" chuds out there 3d printing guns, but there's plenty of those people with guns they bought legally as well. The biggest large scale ghost gun manufacturing I've heard about is the Burmese resistance fighters who have been printing en masse a design to fight back against a genocidal military coup in Myanmar because the international community has largely ignored what's going on there and they can't get guns any other way, which is exactly the sort of situation that design was created for.

But for that you need all the parts, including the gunpowder. And for the gunpowder you need a special permission for handling explosives, at least in my country. So you just move into areas of even more difficult to procure things. it seems far easier to just buy a gun and ammunition somewhere else and take it over the border into California.