First I'll assume you already reload shotgun shells. Many guys shoot 2 3/4'' shells in 2 1/2" chambers. Pressures will only go up a couple of hundred pounds. But if you want to reload 2 1/2' that's fine. I've talked to the guys at Claybuster wads and he said if it was him, he wouldn't worry about using wads meant for say 1 1/4 oz loads with a formula for 1 or 7/8oz loads if the loads were down in the 7500psi and under. That way they would fit in 2 1/2" shells. Any data for 2 3/4 will work for 2 1/2 if you can get everything to fit. That's where the shorter cushion wad comes into play. In my Fed papers cut to 2 1/2 I use the CB-3118 wad meant for 1 1/4oz but load 7/8oz of shot, 16.5grs of 700X and a mild primer like Win or Cheddite will give 6500psi and 1150fps. In a Rem hull it's 7600psi. You could also substitute fiber wads for the correct load height [ for a good crimp ] and have even less pressure. Fiber wads will usually give about 10% less pressure, not always, but never more pressure. Go online to any of the powder companies for load data for the 2 3/4 and feel safe in using it. You'll just have to figure out a way to make it fit. One more option is to use a roll crimp - it takes about a 1/4' less shell to make the crimp.
https://www.precisionreloading.com/ has the best roll crimp tolls that go in a drill. Good luck. Oh, my keyboard took a crap and prints only the foot symbol and not the inch symbol.
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