Ok yeah it doesn't matter what we proof it with just to get an idea where it will start failing in fact maybe we could even use a press to load a locked bolt in a trunnion to even simulate bolt thrust might be as good as loading a proof load with limited equepment. Which I could do that on my buddies press that has a pressure gauge that also shows how pounds or tons are being applied with the press if the bolt and trunnion can handle 5 tons (10,000 lbs) then I think that would be a great sign especially since a round being fired is maybe less than a second while a press is continuous though I wouldn't necesarily leave it loaded too increadibly long. Does that sound like it would be a good test?
By the way what kind of pressure do factory load .243's run? I would love a .243 like you're doing but I'm not sure I want any rifle that can only eat hand loads.
I have been thinking about wildcats for a while like say a 6x57 but loaded to match .243 performance with the larger case capacity it might keep pressure lower what do you think?