Military auto shotguns have been dabbled with for a while, and one of the more interesting projects was the CAWS project. Special shotgun rounds, special shotgun. It never really got off the ground but you can see some DNA was passed down to the Daewoo, USAS, Atchisson and a few others.
I loves me some auto shotgun action as much as the next guy, but I think the basic pumpgun is probably a better choice. On the other hand, I wouldnt turn down a USAS-12 if one came my way. (And, yeah, the fedgoons messed up my fun by making it a Destructive Device.)