If it's as simple as looking at a chart, or data from 1912, why can I load a 20 ga. load that achieves the same velocity, with the same payload, with less pressure than some equivalent 12 ga. loads?

I know and understand, the answer has to do with burning rates of different powders. So, wouldn't any accurate answer to HB's query have to include that information as a caveat? I did not understand him to be asking about max SAAMI pressures, but maybe I'm mistaken.

May God bless America and those who defend her.