"3d printed guns" aren't all homebrew. Usually with guns you can just buy literally every part except for a certain part that's serialized and legally considered to be "the gun" (ship of Theseus style; any other part can be replaced and it's the same gun, but if that one's different it's a different gun)

It's pretty trivial to print only that part and buy the rest. Also completely legal in many states, here's a website with a bunch of examples:

https://www.printyour2a.com/frames