Accurate Arms Handloading book, with a red cover, possibly the 1990's edition, listed black powder loads in all of the Sharps ctgs.
With the normal BP loadings of 120gr. in the .45 3 1/4" case and the .50 in 3 1/4" cases, both rifles developed between 28,000 and 30,000PSI (pezio). These were with 500gr. bullets though.
Even the "lowly" .45/70 black powder round using 70gr. BP and 405gr. lead bullet developed 22,000psi.