RE: What is a good duck gun?
Typically all new shotguns that are designed for waterfowl hunting in mind are chambered for 3" or 3 1/2" these days. I wouldn't buy a shotgun for hunting waterfowl or pheasants chambered less than 3". There is so much more selection of ammo for these in waterfowl loads, too. Unless you load your own, you may have a tough time finding any variety in 2 3/4" waterfowl loads.
I suppose you've already gotten the shotgun. If not, I'd recommend taking a hard look at Benelli M2 (3") or Benelli Super Black Eagle II (3 1/2") Both are terrific, reliable shotguns with the inertia bolt ejection system. They will both serve you well in all types of weather and conditions.
Anyway, have fun in your new sport.