Hmmmm... one of those old market-hunter's shotguns? They were prodigous monsters. Something like an four-bore, which would hold four pounds of shot, would probably do it. The trick is in the shot selection.

If you ever happen to be in Maryland, there are some big market guns at the old Decoy Museum in Havre de Grace. The hunters would mount them on pintles that were bolted to the gunwales of their skiffs. They'd row out into a raft of sleeping ducks, and just blow the whole flock to kingdom come.