doublegunfan:
You will need to load over the maximum recommended load to prove the gun. General concensus is to achieve a pressure overload of around 25% above maximum factory load. British proof achieves this in medium-bore cartridges by loading a 10% heavier projectile over the same powder charge. You must measure all the critical dimensions, then fire the proof-loads, one in each barrel will do, then measure everything again. If there are no dimensional changes, the gun has survived proof.
A 'cheat' which is often used in Australia, where we have no proof laws or proof-house, is to heavily grease the factory-round with case-lube, then fire it in the rifle. This is generally considered to increase the back-thrust by 15 to 20%, but does not increase pressure as such. Fine for testing the actions of bolt-guns where barrel-wall thickness at the chamber is not an issue, but not very definitive for a double rifle on a shotgun action.
Do you have Ellis Brown's book? He gives some pages to proof-testing...
