[Paul Harm]

You didn't say what shell you're using. Remington and Winchester are a tapered wall and have less space where as Federal and all the imports are a straight walled shell and have more room. If you look up reloading data you'll notice the SW shells usually take another grain or two of powder for the same pressure. Also Federal and CCI 209M are magnum primers, hence they're hotter giving more pressure. Usually there's less powder when going to the hot primers. So, IMHO data for 2 1/2" shells is the same as for 2 3/4" shells except you have to find a way to make everything fit in the shorter shell. One way is to substitute a wad, from the same manufacture, that has a shorter cushion. Use a wad intended for 1 1/8oz but load 7/8oz. I called Claybusters and ask if I was loading low pressure shells and did a sub like I just suggested would the load be safe. He said he didn't have any data, but if was him, he wouldn't be afraid to shoot them. So I use a wad meant for 1 1/4 but use 1oz. The shot sits down in the cup and the petals stick up and fold over with the crimp. Or you could use fiber wads and adjust the total load column by using different cushion wads as Carl suggested. In my IMR manual with a Fed shell, 1oz of shot, and any primer, 17grs gives 1200fps and 7000psi. In a Rem or Win shell, 15grs has 7000psi but only 1100fps. This all with plastic wads. Using fiber wads will lower pressure with the same load. You could go a grain up in powder and not worry. These are also with a fold crimp. A roll crimp will lower pressures slightly. You didn't say why you want a 2 1/2" shell. A gun chambered for 2 1/2 can shoot 2 3/4 shells and only raise pressure about 500 to 800psi. If you're loading under 7000psi I wouldn't worry about pressure.