Very similar to modern pressures when adding 10-14%, and the 20g far exceeds the SAAMI recommended max. of 11,500 psi

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As usual, Doc Drew, excellent historical information. However, you slipped on the current SAAMI recommended max pressure. Current max avg pressure figure for the 20ga is 12,000 psi. So the vintage loads were actually higher than the current standard . . . but not by quite as much as you indicated..