As I recall, some folks got in trouble for this years back. You are supposed to do it yourself, without engaging a commercial entity or something because they effectively become a manufacturer. That said, there are folks on this board that have a tremendous amount of machining experience, and I am sure they can point you in the right direction while still keeping everything legal.
I personally know someone that got in trouble for renting his mill out. He was selling the paperweights as well, which may have had something to do with it. But the ATF sent an undercover guy there, the undercover guy did all of the work while the guy I know was telling him what to do.
I guess what caused the problem was at the end, shop owner took it back and "cleaned it up".