I use Cast Performance bullets in a .480 Ruger pistol. They are hard and ,if the flat meplat type, will do a good job on a bison. I killed one with a flat meplat, hard-cast lead bullet (not a CP) out of a .50 muzzleloader ( .45 cal. in a sabot). Medium- sized younger bull @ around 75 yards. Shot through the lungs, he lasted about 20 seconds. That was after we chased them on foot about 2 miles into a canyon. I couldn't get reloaded before he expired. Can't help with a load, but IMO your velocity parameters are good. I was in the neighborhood of 1600 fps in the M/L.