Why not just use 2 1/2" shells in 2 1/2" marked shotguns? They are readily available, same with hulls and components. Even discarded inexpensive 2 3/4" Euro hulls can be had for free and cut down. Mec makes adaptors for their reloaders to load 2 1/2" shells. I have Mec's in 16ga and 12ga set up for 2 1/2"
I don't even bother with 2 3/4" shells any longer, 2 1/2" shells work just fine in my 2 3/4" guns and I never have to sort through shells to make sure I have the correct ones. Even all my waterfowl loads are in 2 1/2" using ITX or Bismuth. The only 2 3/4" loads I do any longer are 1 1/4 ounce coyote loads for my combo guns.
I doubt there are many 2 1/2" chambered shotguns used for really high volume shooting where loading or buying 2 1/2" shells would be problematic.
The flip side of the above is also true. If you reload 2 3/4" American hulls to appropriate pressures, you have no need to buy 2 1/2" shells. But why bother, since 2 1/2" factory shells are indeed available . . . assuming you have them on hand before you head to Bowbells, North Dakota for your next hunt--where I guarantee you won't find them at the local Wal-Mart (assuming there is one). So, in either case, you need to have appropriate ammo on hand, and you can do it either by buying 2 1/2" factory shells . . . or by making your own by using readily available 2 3/4" hulls.
The advantage of making your own: If you shoot your 2 1/2" gun very much--and I've put thousands of rounds through various 2 1/2" guns over the years--you will save enough money to buy yourself another 2 1/2" gun (or two). Reloading for the 12ga in particular, it's very easy to achieve low pressures, and it's very inexpensive.
Take your pick.
