My Lyman "BLACK POWDER HANDBOOK & LOADING MANUAL" 2nd edition, shows load data for a 310 grain Buffalo Ball-et in a .54 caliber, 8 inch pistol. (p 292)
Using GOEX 3Fg powder, the loads they list vary from 35 grains to 50 grains. The muzzle velocity varies from 838 fps to 952 fps and the energy from 484 to 624 pound feet. I notice the chamber pressures vary from 7,800 psi to 10,100 psi. IMO, that's a lot of pressure.
If I was shooting a single shot pistol I would keep the powder load to 40 grains. That gave a velocity of 889 fps and an energy of 544 pound feet. More than enough for anything a pistol needs to do.