Increasing the chamber length on American guns isn't wise either, for the same reason. Short chambered American guns were also originally proofed at pressures lower than 2 3/4" guns--or at least those that were produced after the appearance of the Super-X shell, which pretty much drove the conversion to a 2 3/4" standard. It's likely to be more of an issue with Brit guns than American if we're talking 12's, because American 12's tended to be built stouter than your average Brit 12. But if we're talking smallbores, then you can run into problems with American guns as well. Especially 20's, and especially if someone decides that a 2 1/2" 20 should grow to 3".
L. Brown and Roy,
I wrestled with the same issue. No, I didn't end up lengthening the chamber; instead I sold the gun. Are you suggesting sticking with 2 1/2" shells? Or are you suggesting that using 2 3/4" low-pressure (whatever that means) shells in a 2 1/2" chamber is superior to chamber lengthening? Rest assured I'm asking with genuine sincerity and not poking a wasp's nest. Thanks.
