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.
The easiest--although it gets somewhat costly if you shoot the gun a lot--is to use factory short shells. But yes, using low pressure loads in 2 3/4" hulls in a gun with shorter chambers is definitely superior to chamber lengthening. What happens when you lengthen the chamber is that you remove metal where the pressure is still quite high. And then, because you now have a 2 3/4" gun, you're tempted to shoot any 2 3/4" shell in that gun. So you're risking an increase in pressure from the load that is higher pressure than the gun was designed to handle, in a gun that's had metal removed from the chamber. With a 2 3/4" hull loaded to low pressure, you're assured of staying within the service pressure standard for which that gun was designed, if: a) You know what that service pressure is; and b) You build in a safety margin of 2,000 psi or more below that service pressure limit. Bell's experiments with long hulls in short chambers, reported in Double Gun Journal, showed very occasional pressure increases of slightly over 1,000 psi due to the longer hull. Most were under 1,000 psi; some quite a bit under. So, for example, if you know that your gun's designed service pressure is 10,730 psi--which it is for a current CIP "standard proof" gun--and if that gun has factory 2 1/2" chambers, you stick with loads in the 8000-8500 psi range (or lower). Which is quite easy to do with the 12ga in particular. And you're in good shape.
As pointed out, however, if there should be some accident and if the gunmaker in question is still in business, you're probably out of luck--not so much because you used a 2 3/4" hull in a 2 1/2" chamber, but because it was a reload. I'd expect any gun manufacturer to be very unlikely to pay damage claims based on the use of reloads, even if those shells are the appropriate length. Factory loads, with witnesses . . . that could well be a different story.
