It might be legal if you pay the feds for the permit AND your state allows it.
From the Serbu site......
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What the hell is this NFA stuff? NFA stands for the National Firearms Act, which was passed by Congress in 1934. The NFA provides for the registration and tax of certain types of weapons: machine guns, silencers, short-barreled rifles, sawed-off shotguns and gadget guns (which are considered to be AOWs- Any Other Weapon. Obviously, the BFG-50 rifle is NOT an NFA weapon, though there are bills in Congress working to change that). The popular story is that the NFA was passed to help curb the violence of the "Roaring '20s", brought about mostly because of Prohibition. Anyone who knows anything about the way our Federal Government works will realize that with the ending of Prohibition in 1933, something had to be done with all those Treasury agents, and enforcing a new batch of bogus laws was just the ticket. The $200 tax levied by the NFA ($5 for AOWs) was, in 1934, an incredibly large sum. Now it's mostly an annoyance. The bottom line is that yes, you can buy a machine gun, silencer, or other similarly evil weapon (that's a joke, of course...evil is not a trait which objects can possess), provided that you live in a state which allows them. You'll have to jump through a few hoops and pay the tax, but at least you can still buy this stuff.
http://www.serbu.com/legacy/nfainfo.htm