It is true that per Sherman Bell's tests the pressure increase due to the longer case was not all that great. Average 738 psi. However, there was one shell tested that resulted in an increase of 1,216 psi. And if you're starting with a factory shell made to 2 3/4" pressure standards (figure maybe another 1,000 psi), then it's possible you could run into trouble shooting such loads in 2 1/2" chambers. But if you know what you're doing and if the gun is in good condition and safe to shoot to start with, you can certainly make up reloads in 2 3/4" hulls that will be perfectly safe in guns with 2 1/2" chambers. The only exception would be those guns--usually dating back to the 19th century--that have very short and sharply tapered forcing cones. In some of those, even British/European shells made up in 2 3/4" hulls but specifically for use in 2 1/2" chambers shouldn't be used. You'll usually know right away: Increased recoil, and/or the end blown off the hull. In those guns, only true 2 1/2" shells should be used.
