I loaded for my bro, a 300gr. Hornady jacketed bullet ahead of foam plug and 62gr. of RE#7 for his Shiloh Sharps in .45 3 1/4" having a 30" bl.
His velocity was 2,200fps & this load cloverleafed at 100 yards using his tang sight and hooded globe front sight with aperture.
He shot an immature bullet moose with it in the 1980's, broadside and the bullet the off side exited making a 1" diameter hole. The moose walked 20' and fell down to stay.

I don't know if this would be a load for that old H&H, however due to the length of the case, it most likely is very close to a maximum load.

The black powder firing pin in this rifle worked perfectly and showed same disruption as a 100gr. load of black powder and the same bullet, along with card wads to fill the space, showed almost identical accuracy as well.