If geese are in your future, you'll want a magnum. 3 1/2" loads carry enough T shot, otherwise heavy shot in 3" works great, but is expensive. Winchester Xpert Hi Velocity number 3 steel in 2 3/4" works fine for me on big ducks. Geese are tougher and require bigger shot and 3" or 3.5" has more of that bigger shot in the pattern than 2 3/4" does.
As to guns, any of the above are fine shotguns. I'm thinkin' about getting a 935 Mossberg, myself. For the money, the Remington Spartan 453 is a good, functional gun for waterfowl. Shoots 2 3/4 to 3 1/2" loads. The Spartan is gaining fans big time in the duck marshes. It's a heavy gun for all around use, but for waterfowling, that's a bonus to soak up that recoil. It might be a little slow on the swing, though, for those early morning teal buzzing in the first few minutes of shooting time.
I've been using a pump M500 mossberg on geese and ducks forever alternately with a Winchester M1400 auto on ducks and doves. I really ain't in a big huff for a new shotgun as the ones I have get the job done well, frankly. The camo finish on my Mossberg is desirable down here in the salt marshes and bays, very rust resistant. I took a tip from SM and started using paste wax on my blue/wood 1400 and it's holding up well enough and I'm impressed enough with this idea that I've started shooting it more on ducks. It's a sweetheart to shoot, very light on the shoulder with heavy loads. It's a gas operated semi-auto. And, it fit me perfectly right out of the box. I hunt geese maybe a couple times a year and the Mossberg does a great job with 3" hevi shot. If I hunted geese more, I'd want a 3.5" to shoot steel T, cheaper ammo and effective. I'm going to try Remington hevi steel this coming season, though. It's cheaper than hevi shot and supposed to be pretty good. We'll see.
