Ted, I haven't shot Eleys in some time and I don't remember for sure going back to when I did use them fairly frequently. But
SOME British shells are actually 67-67.5MM, which is slightly longer than 2 1/2", and are loaded to 2 1/2" pressures and marked as appropriate for use in 2 1/2" guns. Several years ago, Charles Fergus reported (SSM article) that he was experiencing blown ends AND increased recoil from those loads in a 19th century Brit gun with short, sharp forcing cones. Same loads worked fine in a between the wars Brit gun, also 2 1/2", but with longer tapered forcing cones. He went to true 2 1/2" shells in his 19th century Brit gun: No more issues with blown ends or increased recoil.
Both MAJ Burrard and Gough Thomas reported that the Brits also loaded 2 3/4" hulls to 2 1/2" pressures, and that those were OK--and showed no pressure increases--when fired in guns with 2 1/2" chambers. However, per Fergus--and there have been a few similar reports--what holds in more modern Brit guns with 2 1/2" chambers does not hold in ALL 2 1/2" chambered guns. I expect the majority of us here with 2 1/2" guns through which we shoot a lot of shells are reloading 2 3/4" American hulls to appropriate pressures. I've done so on a number of Brit doubles with no problems, but they've all been 20th century guns with "normal" forcing cones. If you're blowing the ends off hulls, and in particular if you're getting increased recoil, that's a really good time to stop what you're doing and go to true 2 1/2" hulls.
