Non-sporting style shotguns (i.e. USAS, Street Sweeper, etc) are classified as destructive devices b/c they have bore(s) larger than .5", are fired by an "explosive charge" shotgun shell and they are not (for the most part) meant to be fired from the shoulder.
The BATF definitions of pistolgripped only shotgun and destructive device are pretty similar. There is a good article about this in the latest SAR in the legal column.
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