An equation for proof pressure per the Oktober 30th, 1897 Belgian Decree/May 10th, 1898 note the Proof Charges were to attain a pressure of 80% over the maximum service pressure and the resulting mark was the Rampant Lion over the Max load stamp.
Belgian Decree of November 18th, 1903 with the >>Unique<< Powder for the 4th proof was not to exceed 875 kg/cm^2 or 13k lbs/in^2.
The Pressure for the 5th Proof Effort was not to exceed 750 kg/cm^2 or 11k lbs/in^2.
A little tid-bit more of info from another source gives that early semi-smokeless powders achieved a vast array of pressures between 6k to 10k PSI. So it was the goal of the Oktober 30th, 1897 Belgian Decree to adopt a powder to achieve a proof pressure of 80% over the max service pressure but that value must exceed 8.8k PSI.
Hochachtungsvoll,
Raimey
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