It is my understanding that the 1187 Sporting Clays gun is simply the 1187 with a barrel chambered for only 2 3/4 inch shells. I don't think you'll find a 1187 Sporting Clays barrel chambered for 3" shells or else it wouldn't be called a "Sporting Clays" gun since you don't need 3" shells in sporting clays.
Remington has now gone back to making the 1100 which is chambered for the 2 3/4" shells only and the 1187 which is chambered for both 2 3/4" and 3" shells.
I think you can buy a barrel for 3" shells for your 1187, but it won't be called the "Sporting Clays" barrel. Check with the Remington site to be sure, but I think I'm right.
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