There seems to be an understandable confusion among many posters as to the use of proof terms as a historical point of reference and the way that modern ammunition manufacturers cater for these.

A British gun stamped 2 1/2" or 65mm or with a diamond with a 'C' inside it (which mark you get depends on the age of the gun and when it was proofed)is designed for a 'normal' game load.

It is proofed (and in some years also stamped) with a service pressure of 3 tons per square inch. You need to consider the 2 1/2" chamber length, because you do not want the end of the crimp entering the forcing cone, failing to open fully and therefore increasing pressure. Ammunition in cases of 65mm, 67mm and 67.5mm is olny allowed to develop pressures in keeping with guns proofed at this (lowest) proof test.

BUT

You also need to be aware that modern 70mm cased shells are allowed to develop higher pressures because a gun stamped as proofed for 2 3/4" shells will have longer chamber, thereby acccommodating the crimp and allowing it to open fully but also because the proof pressure for these guns is done to 3 1/4 tons per square inch.

Therefore, a 24g 70mm shell may have a lower pellet payload than a 30g 65mm shell but it may well produce higher pressures.

In the UK, this is common knowledge and no shooter would put 70mm cased shells in his 2 1/2" game gun any more than he would put diesel in his motorcycle.

A few cases in poit:
My 1889 Purdey - has 2 1/2" chambers and is proof tested to 3 tons per square inch. In it I regularly and safely use the following:

34g bismuth shot loads 'Eley Maximum' brand in 67mm cases in boxes marked 'Suitable for guns proofed for 2 1/2" shells'.

30g Game Bore Pure Gold lead loads in 65mm cases marked as above.

24g Express HV lead loads in 65mm cases,marked as above.

I do not use:

White Gold 28g lead in 70mm cases marked 'suitable for use in guns proved for 2 3/4 inch shells' because although they are lower in pellet weight, they are more powerfully charged and the cases will not open fully in the chambers of my gun.

That same goes for any case marked 70mm or 2 3/4".

I hope that helps a bit - and I apologise for the grumpy tone of my earlier (early morning pre-coffee) message - I stand by what I said but I should have worded it more respectfully.

www.vintageguns.co.uk