Go to Accurate Arms web site for loading data on the Sharps black powder ctg. for the same length as English rounds. They will show what velocities they got from black powder equivalent loads with the powders they used. Their standard for the .50 3-1/4" was 28,000CUP, if I'm not mistaken. Depending on the bullet weight, this low pressure allowed them to load the .50 to well over 4,000FPE, whatever that number actually means.
: The smaller bores require higher pressures to obtain the same FPE - that's just common sense.