If you re-chamber it to 2 1/2" it would still be out of proof as the proof becomes invalid when the barrels are materially altered - this includes chamber liners. I re-proof guns routinely - we generally re-proof all guns for 70mm now, the proof pressure are the same for 65mm and 70mm and lengthening the chambers makes the options a bit easier, especially for those travelling.
Dig, somewhat confused by the above. As I understand it, 65mm guns can only be standard proof (the old 850 bars). 70mm guns can be standard proof, but you can also request magnum (now Superior) proof, can you not? And if the gun is standard proof, even if 70mm, the service pressure is lower (10,730 psi) than SAAMI standard service pressure (11,500 psi) here in the States. Thus, would not a 70mm standard proof gun still have issues with American factory 2 3/4" shells?
