Good catch, Miller. I should have said "If you're getting significantly increased recoil and/or blowing the ends off shells . . . " Goes to show you that all of us can use help from sharp readers (or editors who understand the subject).

That being said, I don't think the issue addressed by Fergus and Thomas has anything to do with the loaded length of the shell extending into the cone. Unfortunately, the only unfired Brit shells I have at present are true 2 1/2". However, I have a fired 67MM Game Bore 16ga, and it is exactly the same length as a fired Remington 16ga Game Load. Both crimped, so it seems to me their unfired length would also have to be the same. And while I agree with you that an unfired shell extending into the cone should be avoided (that's almost certainly what happened when Bell touched off a 3" shell in a 2 1/2" chamber), the only way that could be the issue in the cases reported by Fergus and Thomas is if the guns in question (in which the ends were blown and sharper recoil reported) had chambers shorter than 2 1/2". We know that the 67MM shells, which the Brits say right on the shell boxes are approved for use in guns with 2 1/2" chambers, do in fact function just fine in MOST guns with 2 1/2" chambers. So my strong inclination is to agree with Thomas and Fergus that the culprit is the length and taper of the forcing cone, not the length of the chamber--although it would be interesting to measure the chambers on those guns in which problems have been reported, then do the same with other 2 1/2" guns that handle the same shells with no problems.

I suppose it could be that Brit gunmakers prior to 1900--like some of their American counterparts a few decades later--were intentionally short-chambering their "2 1/2" guns, and for the same reason: in order to get improved patterns. That'd work fine as long as the shells were true 2 1/2", but might indeed result in the unfired shell extending into the cone if the chamber were somewhat shorter than 2 1/2". Haven't ever read anything indicating the Brits adopted that practice, but since some American gunmakers did, I guess you never know.