Geese require BIGGER shot sizes in steel than lead. Bigger shot, heavier, carries more energy down range. It is about getting enough of that big shot in a shell to pattern. Ts are big, 20 caliber or something like that, nearly 4 buck in size. It takes 3 1/2" of shell to get enough of it in a shell. Oh, sure, if you wanna pay 3.50 a round for hevi shot, knock yourself out. I know 3" hevi shot BB works great, but it depletes my wallet and an alarming rate.
On other threads like this (do a search) I've stated that the new 2.75" fast steel 3s or 2s, the 1550 fps stuff, is deadly on ducks. I don't shoot 3" on ducks anymore. I save money on the 2 3/4" stuff and my Winchester auto only chambers 2 3/4" stuff If I decide I wanna shoot that gun.
I shoot geese a couple times a season max. Most of my waterfowling is ducks. I shoot ducks usually with a Mossberg 500 camo, 2 3/4" Winchester high speed Xpert in 3s or 2s. But, I've got a 10 gauge for goose hunting now and throw steel Ts at 'em. That stuff don't mess around. It's good for 60 yards easily on a snow goose and will shoot right through one and leave an exit wound at 30. No more cripples at 35 yards or geese flying off to die somewhere else from infection.
Understand this, it was the Mossberg 835 that was designed for the 3.5" shell early on for goose hunters when steel shot was mandated throughout the land in the early 80s. Steel shot CAUSED, steel shot NECESSITATED the 3.5" shell for the big steel T and F shot that came out as an early answer to the obvious problem of steel shot on big birds. Bismuth and Hevishot came out as an alternative later, but the stuff will run you bankrupt. Steel shot laws also revived the 10 gauge from among the ranks of obsolete historical gauges like 2 and 4 gauge, too, and then the Ultimag came out for the hunter that didn't really want a dedicated 10 gauge goose gun, but wanted an all around gun. The 3.5" 12 gauge Ultimag is good, but if you like goose hunting, I still prefer how the 10 patterns. I just figured I already had enough 12s, why not get a ten to move up? I'll probably get a better one in the future. Mine's an H&R, but it works fine as a goose gun, just would prefer a BPS 10 eventually.