There are many published BP loads in catalogues, cartridge lists, and ballistic tables of the day. The standard .577 BP load was a 530 or 540 gr paper-patched lead bullet over 165gr BP. The widely-published velocity for that load was 1650 fps. Replacing the bullet with a 650-grainer might be expected to require more powder to reach the same velocity, but maybe not. It would certainly increase pressure a bit.

I load 'nitro-for-black' to 1650 fps with a 650gr jacketed bullet, in a Greener double so-chambered, but have no experience with black powder in that cartridge.

Six drams is closer to 165 than 160 gr, BTW.