Buzz, the pressures for 2 1/2" proof are exactly the same as for 2 3/4" proof. The only difference between the two proofs is the length of the chamber. If I had a barrel failure in the critical area where the chamber has been lengthened, I would of course blame the lengthened chambers but that has never happened to me. Failures have to date all been rivelling of the central part of the tube (often in areas of less than ideal MWT) or bulges at the choke forcing cones (usually with choke constriction of over 0.020"). Once or twice a weak action has come off face but this has usually been a thin, early breech loader such as a converted pinfire. A couple of times the lumps have come loose and had had to be re-brazed.
L.Brown, This gun was heavily pitted but with very good MWT's and substantial metal in the chamber area. To remove the pitting, the barrels had to be lapped to beyond their proof size so it HAD to be re-proofed. Given the nature of the 2 1/2" and 2 3/4" proof tests (see above), it was a no-brainer to lengthen the chambers. So when the gun was submitted for proof, it had 70mm chambers, the gun would have been measured at 'view' and when the gun failed (or passed) the paperwork would record it as a 12/70. It would not have to have ever passed 70mm proof to be ticketed as such.
Actually, all proof is done at the 'Standard' level for the chamber length UNLESS you request 'Superior'.
Craigd, The reason why proof is valued by most shooters in the CIP area is that we CAN go into a gun shop and if the box of cartridges says it is suitable for the chamber length of our gun, we can use it with impunity. Obviously, once the gun leaves the CIP area, eg goes to the USA, there is no such standard and ignorance of what a suitable load might be could get someone into trouble. I make no criticism of the situation in the USA, I just try to manage the risk as best I can.
