nh: a 3 in. bdl will not work on a 2 3/4 in recever !!!

  3 in. shells are good , but you can get away with 2 3/4 " if you dont sky blast.. wait for the shot .

 are you going to be doing shooting over decoys ??? if you are just wait till the get in range and set their wings...

 you can also get some high power 2 3/4 mag. shell. but you do know that they have to be STEEL SHOT. then you have to watch out what choke you shoot it with ....go to the remington site i think they have a chart on what size steel shot to use with what choke. they dont use reguler shot size for steel.

There are many things available to you now to make 2 3/4 in. shells effective on ducks.  The Remington hevi-shot load is absolutely amazing as far as effectiveness on ducks and geese.   It is spendy but if you are a half ways decent shot you will kill just as many ducks as you partner and once you give them a dose of hevi-shot they are down for good usually. 

If hevi-shot isn't in your budget look at some of the other premium brands of shotgun shells.  Bismuth etc are all improvement over steel.   The reason 3 in and 3 1/2 in mags are even in use is a response to steel loads.  Hunters were trying to pack more shot into shells in order to kill waterfowl efficiently.  2 3/4 shells were the standard for years before non lead shot was required. 

I disagree with d4570 on the full choke idea.   Before you use any choke/load combo go out and pattern it at 25 and 35 yards.  Your 870 mostly will perform much better with either Improved Cylinder or Modified choke vs. full choke when you are using non lead shot.  Regardless go out and pattern your gun and load to find the best pattern. 

Nemont

I agree totally with nemont, great advise.  I would emphasize not using a full choke with steel.  Very bad idea.

Now nemont, find a new avatar.

2 3/4 inch shells are just as effective as 3" IMO.  I also use 20 guage 2 3/4 inch for ducks.  with steel a modified or even an improved cylinder is very good over decoys.

I think you have the right shotgun, and the shells are cheaper.  Keep your shots to reasonably distances.

Now, that said. It is a very easy conversion of your 2 3/4 inch to a 3 inch, if you want to go that way.  the action needs only have a new ejector installed; then you can get 3 inch barrel or have the present one re-chambered.  Cost are about $75 locally.  Same can be done with the 1100, but the gas port needs some changes also, still minimal.

BEAR

NH, a lot of the same holds true for shotguns as well as rifles....pick the shot...shot placement....practice...practice....practice.

As a kid growing up in the late 50s and 60s I started hunting with a single 16 stevens and #6 lead shot and I didn't have any trouble sneaking shots at ducks and waiting for the early flights of wood ducks that arrive just as the sun hits the water in the marsh. I got my share of woodies then. In late highschool, I bought a win m12 in duckhunter model. 3"mag-30' full choke barrel. I still used #6 lead shot and a lot of #71/2, all 23/4",and still had my share of downed ducks. #71/2s were deadly on close pass shooting and finishing off cripples. I got caught up in the magnum hype after college but never shot more ducks than I did with the 23/4" shells. If you can see the green head and orange feet with wings cupped, 23/4" shells will work just fine.

If you feel the need for range you can try some of the newer heavy composite shot shells, a bit pricy but they provide a bit extra distance.

I have of late, got back into late season duck sneek/jump hunting after I get my deer. These are usually very wary blacks and mallards with some geese here and there. My success ratio took a big jump when I switched fron #2 to BB shot. I bought some of the surplus winter camo suits sold for $15 dollars ,or so, and have had a blast using it. It is possible to get too close to ducks wearing it.  I highly reccommend carrying a few BBB or T shot in case you chance upon some geese. There is a tremendous thrill in crawling 100 - 200 yards to get within easy range, jumping up and getting close shots as the ducks go practically vertical with suprise.

In my younger days I thought nothing of stripping down and wading armpit deep in river water with shell ice flowing by to snag in a duck I had just shot further upriver.

I was a duck hunting fanatic back then...as you can probably tell from this gushy posting. If I applied myself to my studying as I did to duck hunting I'd be typing this from one of my hunting lodges around the world.