That photograph Drewe posted of the two cartridge cases came from me, this was a start of a problem that started to become more common here with some cartridge makers.The cartridge case on the left was an Eley "Blue" 67 mm this cartridge rattled my teeth because it improved the recoil no end. The other cartridge case is my usual go to Hull 65 mm and as you can see they do not agree on size. I have a number of guns that are 21/2 inch cambers and they are exactly that size, the guns where built before 1880. I did contact Eley at the time their reply was it was within accepted specifications. If I lived on your side of the pond this would not be a problem just extend the chamber to accept 67 mm on each of the veteran guns BUT!! If that is done on this side of the pond the guns would then be out of proof and required to go through proof again, as each gun is in proof there is that chance that a couple of costly gun's could end up in pieces after test firing I will give that a miss. At that time it was a sort of Russian Roulette when purchasing 21/2 inch cartridges what case length you would end up with.
