Charles Fergus reported something along these lines with two Brit guns he owned. Both 2 1/2", one late 19th century with short, sharply angled cones; the other 1930's with longer cones. 2 1/2" shells were fine in both. However, the longer Brit ammo (67.5 MM, I think) loaded for use in 2 1/2" guns resulted in the ends blown off the cases and increased recoil in the older gun; no problems in the newer one. Don't know whether those shells were getting past the cones on the older gun, but something was causing very different results in one compared to the other.