Unless you have a .22bolt and a can. Way better for ducks then shotguns, just not legal.Not gonna do much duck hunting with an AR...but I'm not a wing shooter anyway.
Remington 870 or 1187, in 12 or 20, is the easy answer.
The question gets much harder if you say, "one caliber, no shotgun." It would have to be something deer legal that could still, in a pinch, be used to take bunnies and squirrels without completely destroying them. Maybe a .300 blkt?