Ah, you need to read up, Lewis
"and it will remain just a piece of metal which you can send via post, Fedex etc to anyone you like."
No, that's against the law. To transfer this to someone else, you must first obtain the right licenses, stamp the thing with a serial number, and then it can be transferred after the paperwork is done. This is the part with the serial number, and this is the part that legally constitutes the weapon.
You can, however, fill in all of the part that you can't mill, like everything else, and happily go legally shooting. That isn't against federal law.
Gun laws vary from state to state. Perhaps a state requires firearms registration, perhaps it doesn't. Perhaps it allows a person to manufacture a firearm for themselves, perhaps it doesn't. Where I live, this would be perfectly legal, but I'm not so sure about New Jersey. Also, state laws may restrict purchase to state residents, and other things like that. For instance, in the State of Washington, an individual may not own a full-auto weapon, but that weapon may be owned by a corporation. Go figure.