First off, I'm just a redneck from the backwoods of Pa. not a shotgun expert by any means. I'm just going by what I learned hunting over the years.
Back in the day you had 2 3/4" loads and the 3" magnum. The next step up was a 10 gauge in 3".
The older fellas were killing turkey with 2 3/4" guns cause thats what they had. The 3" magnum was also good for turkeys and better for geese.
Then they changed the laws for hunting waterfowl...no lead.
Steel shot weighs less than lead, and hits with less force. I was shooting geese with 3" steel at 30-40 yards and not killing them. IMHO steel sucks! Now for mallards, wood ducks and other smaller ducks... 3" steel would work.
The 3.5" gun came out, and that worked much better for hunting geese using steel shot.
When Bismuth and Hevi-Shot came out... many of us quit using steel. A 3" load of Hevi-Shot hits harder than a 3.5" loading of steel.
With todays ammo, a 3" gun is plenty for turkeys or geese...unless you are shooting steel shot.
If you can't kill a turkey with a 3" load of #6 shot, you need to quit buying bigger guns and learn how to call turkeys. I have killed 13 ( 5 gobblers) with 2 3/4" and 3" magnums...never a 3.5". All from 20-35 yards.
If you are buying a gun to hunt turkey, geese and small game, along with clays... and you plan on using lead, steel, and Hevi-Shot... then a 3.5" gun makes sense. But if you are not, a 3" gun is all you need. Anything more just costs more and kicks the crap out of you. You can hunt deer with a .338 Win Mag...but I'll stick to my .308.
My HD shotgun is loaded with 2 3/4" 000 buck.
This is just the way I feel about it. Others will disagree, which is why they make so many different guns.
Regards,
Buckmark