I prefer a Light 20. Puts out plenty of lead, but you can shoot it all day without being shoulder-bruised.
Even a .410 would probably be enough. Or my little 28 ga. dove gun . . . Remington 11-48, actually come to think of it it's a classic and too good to waste on that dreck . . .
As you wish ... I can hammer away all day with my lovely new Benelli Nova or lovely old Winchester 1300. Both 12s. This could be a great new sport, though ... use Precious Moments instead of clay pigeons.
Heh heh heh.