isnt that a NFA item? I havent looked into short barreled shotgun, i know sub 16inch rifle barrels make it a SBR. (to be fair, who gets a SBS?)
Someone who wants to use it in close quarters, obviously. And bullpup designs, by moving the action into the butt, result in what is effectively a SBS or SBR without running into the NFA rules.
For that matter, it's not like getting an NFA item is all that difficult. I'm pretty sure if you do a little looking, I've posted a step-by-step guide to doing so. It'll cost you $200 and some paperwork.