For the most part all that is said here is true!!!! You should mike up your water table and for that matter all breech clearances prior to firing your proof loads. After proofing if any have changed then the gun did not work out! Steel is both elastic and plastic in action. What I mean by this is that up to a given pressure steel will compress and spring back, (Only for a given number of repititions, thus stress failure.) or distorte as when fordged. If you take a piece of steel which is properly hardened and beat it with a hammer it will up to a point spring back to shape. If the steel is to hard it may break and if not hardened the same steel will get beaten out of shape. Never hold on to a gun during proof testing as you may not live to check it out!!!!
I use the German methiod which is to take the max load for a given bullet weight/ cartridge and up the bullet weight 10 percent. Thus a 450 grain bullet max load would have a 500 grain bullet seated on to of it. Fire one proof and one standerd load from each barrel and check all tollerances. If any thing has changed then it did not pass proofing.