European proof rules told most of the time: The man submitting a gun for proof has to tell the cartridge for which the gun is to be proofed. If it is a factory chambering, maximum pressure of the service load is determined by the CIP tables, and so is the proofload with 30% more pressure than the service load maximum. At least in Germany, the 1940 proof rule prescribed the maximum pressure of the highest pressure German factory load then available to be the max service cartridge pressure, hence the ridiculously low max pressures of some old cartridges like the 9.5x57M-Sch. But there always was an exception: For cartridges with no standard factory loads available, the maker has to describe exactly the load for which the gun is to be proofed and has to send in at least six such loads and components, perhaps even his reloading tools, with the gun. Then the gun is proofed for this load alone. Loading ammo with a higher pressure than the load submitted for proof is illegal. Remember, proof is meant to determine if a gun is safe using factory cartridges, many handloading "recipes" make the gun "out of proof" for this load and the user is on his own, without the slightest chance to sue anyone.

Proof rules simply require a 30% higher pressure than the service load! If your gun shows any defect such as bolt setback or gaping, it is to be rejected. This indeed troubles the proofhouses with some new American high-intensity cartridges, as their service pressure is already so high that the prescribed 30% increase makes brass "flow" like a liquid and makes properproofing impossible.

BTW, how do you know your "improved" cartridges give such a low back pressure? Have you really measured anything under controlled conditions or are you merely guessing too? How do you arrive at a 100% increase? British experience simply state "more than 30%, likely 50%".