I'm using 12.5grs of unique with a 450gr copy of the government bullet in my sharps with good results and I use to use the same load in a Remington navy model rolling block when I had one. You ought to see that ram jump when one of those big slugs hit it.

Thanks

For vintage rifles I believe you cant beat black powder.
That said in my replica sharps rifle I shoot a duplex smokeless powder load.
Case -  Starline 50-70
bullet - 550gn NEI semi spt. 512" BHN-10 NASA lube
primer - CCI #$ 250 lrm
powder - 1cc Accurate LT-30 priming charge
---------- 36.8gn WC-870 (same as H-870)
Note  The 870 powder is dropped through a 25" drop tube
Bullet is seated to touch the powder to keep them from mixing
A four shot string chrono graphed at,
1209, 1218, 1218, 1209  fps for a 9 fps extreme spread and excellent accuracy.
To be clear though for vintage arms I prefer BP.