On firearms in the US, the receiver (usually) is legally “the gun”. Everything else is parts. In a gun store you sign a 4473 federal form for a gun. You can walk out paying cash for parts with no papers. Mailing a gun requires numerous special procedures. Mailing parts is as simple as mailing anything else.

There have been a lot of 3D printed receivers, aka “the gun” made, and then all the relevant parts added. I don’t know how many, because by its nature the numbers aren’t really tracked, but there is an active hobbiest community for the practice.

This is a modernization of the older practice of building guns at home. Using traditional methods, guns including AR-15s (easy) to AKs (hard) have been built at home from non-gun materials for receivers, and then fitted with parts.

Not that I agree with the panic. It’s silly. As above mentioned, 3D printing is an evolution of the practice not a revolutionary new way to access guns.