Huvius, of course this system was used to test ammo,not rifles. I posted this stuff just to show that the British proofhouses, who also pressure tested ammunition to find the proper proof loads, have a lot of experience with the relations of cases, oil and breech pressures. In the example cited by oupa above, the proofmasters were quite sure the barrels would hold the pressure, but tested the breeching of the rifle. They did not just smear some old oil on the cases. They simply knew from their experince with the ammo testing gun that using the Woolwich system's oil on the cases would increase the pressure on the breech by "at least 30%". And , be assured, European proofmasters have some leeway: If they are sure a gun is safe with the prescribed loads, they let it pass, if not, they will reject it. After firing a "blue pill" load (an American expression, as .30-06 military proofloads had a blue primer), they are to inspect a gun for defects, fi has it gone off face. If a break-open has opened the slightest amount, it will be rejected.