Interesting! You know nothing of the gun and complain that you can't hit a duck with a bullet.
I cringe when I ask this question... You are using shot, aren't you? And if so, you're not running steel shot through your gun I assume. I ask this because "bullet" means slug to me in a shotgun. Other than being VERY dangerous to others within a considerable range of your gun, a slug is of little danger to a flying bird. And if you're shooting steel shot, then I'm very afraid that you no longer have much choke left.
So I assume you meant that you were using appropriate loads for your shotgun... If so, then you must realize that doubles are unique in some ways. Since the tubes are located on a horizontal plane and the breach end is bigger than the muzzle of each tube, these guns are "regulated" to a point where the patterns will cross each other. This isn't a big deal for those of us used to doubles, but it's a factor worth consideration.
Next is patterning. The best way I know to pattern a gun is to shoot it at a known range against a good target. You'll soon find that your gun is as individual as any human being. It will like some loads better than others and no two guns will be exactly alike.
Barrel length and chokes will also be a factor in waterfowl hunting. Longer tubes will enhance your swing to some degree. Chokes will regulate your pattern and keep it to varying degrees of tightness at ranges out beyond 15 yds. or so.
And then comes the final question. Is your gun chambered for modern rounds? Older guns are often chambered for slightly shorter rounds. While the 2 3/4" loads will fire safely enough, you'll experience increased recoil. (Not good for the wrist area of your Elcee.) The increased recoil being an indication of elevated pressure which, while probably not unsafe, could cause your gun to toss poor patterns.
My advice is to enjoy your Elcee, but become familiar with her before you abuse either the gun or yourself. You'll be rewarded many times over for having done this as you have a fine shotgun.