ORIGINAL: dawei
Why don't you just get a 3" gun; take to a competent gunsmith & have him rechamber to 3½"? The pressue, (PSI/CUP); is the same for 2¾", 3", or 3½". I have several friends that did this with zero problems.
This is what I plan to do if I can't find a factory manufactured SxS in 3.5" that I like.
Do you know what model shotguns they are having rechambered?