It's worth noting that a 2 3/4" shell only measures 2 3/4" AFTER IT HAS BEEN FIRED. Back when I first looked into the long shell/short chamber question, I compiled a pretty impressive collection of once fired nominal 2 3/4" hulls. While some of them came pretty close, all measured somewhat shorter than 2 3/4".
It's difficult to find very many vintage American 12's that are as light as many British 2 1/2" 12's. I have a William Evans 2 1/2" 12 made in 1901, with 30" barrels. Weighs 6/5 on my postal scale. One likely reason very few American 12's self-destructed after having chambers lengthened to 2 3/4" is that they were made heavier to start with than British 12's. While I have no interest in rechambering my Evans and submitting it for reproof, there are plenty of British guns that have had their chambers lengthened and have passed reproof.
My Evans, with its unaltered 2 1/2" chambers, closes with ease on once fired Remington Gun Club hulls.
