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http://www.serbu.com/top/superShorty.php
Keep in mind that you would need to keep a copy of your Class III stamp paperwork with it at all times, but I'm sure you could laminate it and fold it up in your pocket if you wanted.
There was a thread about this shotgun here:
http://www.texasshooting.com/TexasCHL_F ... 87&t=16460