If this 3.5" auto gun is going to be strictly a turkey gun... then I'd make it a Mossberg 935.

FWIW.... I think the only game/target/hunting situation where a 3.5" shell is an overwhelming advantage is turkey hunting. There are those who swear by them for waterfowling, and while I see the merit behind it, I don't use them for ducks/geese. But turkeys.... I break out the big ones.