Duck hunting is in my blood....
I have only hunted with steel shot. I have learned what works and what does not.
Now you have to be real honest with yourself: How good of a shot are you? Can you hit the ducks consistently at 40 yards? Can you judge distance accurately and also judge species accurately in the heat of the moment? Judging species is very important because of the variations in size that you will likely see in a hunt. A mallard at 65 yards looks bigger than a teal at 35, one is a shoot situation and the other is not.
Pattern your gun, first thing out of the chute once you pick a load. I used to use 2 3/4", then 3", then 3" high velocity because regular 3" are worse at killing ducks than 2 3/4", low velocity is what nueters the 3". With the 2 3/4" or the 3" high vels it's really a horse apiece, both kill ducks GYD inside of 40 yards with a good tight pattern. 40 yards is the limit though, you run out of pellets in the pattern with large shot (#2's) and smaller shot will not get the job done. With a pattern large enough to be workable for the average Joe the 3" high vels and 2 3/4" shells are petered out completely at 35 yards.
Now with the 3.5" shells you can heave enough big shot and have it going fast enough to get the job done well past 55 yards. With BB's you can kill ducks stone dead at 60 yards, 2 BB pellets will kill a duck but 3 is better only if you can't get 4 pellets on the bird. Follow my drift? The patterns it takes to kill ducks at 50 and 60 yards are EXTREMELY tight, they have to be. A typical 25 yard pattern for my 3.5" launcher is less than 10" across, a ragged hole. I do not shoot ducks inside of 30 yards, there is a real risk of blowing them up. I hunt hard pressured birds on public land with a lot of competition. My average shot is 40 yards. My situation dictates what I need to do so that I can harvest ducks cleanly. As a side note I have also fired over a hundred thousand shotshells in the last 3 years developing the ability to shoot well enough. I expend an average of well under 2 shells per bird in hand because of that practice, had 13 birds with 13 shots in one string last year before a miss found me.
So now you have to decide what your ability will allow you to do. If there is little chance of hitting the birds beyond 35 yards (that would put you in with 95%+ of duckhunters) you can shoot 2 3/4" shells with a tight pattern or you can shoot the 3.5" shells and have a much larger pattern with the same density as the 2 3/4" shells. To a man I have seen the 3.5" guns increase the hit and kill ratio dramatically in the guys I have hunted with. The 3.5" guns make a much larger difference than Bismuth or Hevishot have in everyone I have seen too, the magic pellet crowd have wallets a lot lighter is about all. Give a hardcore hunter that can really shoot some hevi though and all bets are off.
I would definately recommend shooting 3.5" shells exclusively if you can handle the recoil. Shoot #4 shot in a good high velocity load for early season and smaller ducks and #2 for later in the year with heavily plumed bigger ducks. Pattern EVERY single load you buy and get the most from them. Kent "Fasteel", Winchester "Supreme High Velocity" and Federal "Premium High Velocity" are the best choices I have found.
