Sherman Bell, in article titled "Finding Out for Myself" Part V "Long Shells in Short Chambers" (Double Gun Journal, Winter 2001) said that with loads that are sensible in a light 2 1/2 inch gun, chamber pressures were only slightly increased by using 2 3/4 inch shells. He felt there was no reason, related to safety, to lengthen an original 2 1/2 inch chambered gun to shoot 2 3/4 shells, as long as the pressure of the 2 3/4 inch load you use is the same as the pressure of a 2 1/2 inch load.
Drew, we're very close on that recommendation, although I'd modify it just a bit. CIP standard proof service pressure (for 2 1/2" 12's) is 10,730 psi. Bell's tests do show increases of greater than 1,000 psi in a few cases, with a 2 3/4" hull being used in a short chamber vs in a 2 3/4" chamber. I'd recommend building in more of a safety cushion than the approx. 800 psi difference between CIP and SAAMI service pressures. Something more on the order of 1500-2000 psi. It's easy enough to find recipes that will keep pressure under 9,000 psi, so that even a 1,500 psi increase due to the longer hull--and that's a little more than Bell ever reported--will still keep you within CIP limits.
