Depending on your jurisdiction, there is nothing illegal about buying an NFA-registered $5-tax 12ga AOW either. Heck, I had one on my NY CCW permit. Few things say “government certified good guy” like a Form 4.
Buying one, if you want your name in a federal weapons registry, is a $5 tax. Making one is a whole lot more expensive penalty (without the license).
But I don’t find the people selling the lawful versions very affordable.
http://www.serbu.com/top/superShorty.php