I did. One of about 30 articles I read today. What I was after was Black Powder service pressures not just safe Black or nitro for Black loads. There are several excellent articles and extracts from The internet on this plus Drew has assembled a massive amount of period data. All well documented by the way.
But I still have not seen a direct standard service pressure for Black powder. For Nitro 850 bars or 1200 bars, 2 1/2, 3, 3 1/2 & 4 tons there are clearly recommended service pressures expressed in bars and psi. Perhaps none exist because pressures on Black were pesio or LUPs and never directly measured in bars , tons or psi. There are formulas to convert LUP o PSI but they just give you an approximate value. Since Black is still offered as a lesser Proof these days there is no great demand for it I’m sure other than for guns which are deemed not like to to pass modern Nitro proof.
One article used a 200% Proof which correlates with a 3 1/4 dram (standard) being proofed at 6 1/2 drams. But the proof pressure is not a linear curve and just taking 1/2 proof pressure as your service pressure could be just a WAG. Most likely that is as close to an answer as I will get.
