Here's an article on a short barreled .577 Howdah.

https://www.blackpowdercartridge.com/the-577-howdah-pistol

"With 70 grains of FFg, velocity was over 1,000 fps with a round ball(approx.300gr.) and 775 fps with a conical bullet(440gr.)"

From a Pedersoli 20 bore with round ball of about 320gr.,
from clean barrel:
55gr. Phony powder - 826fps
60gr. " - 843fps
70gr. " - 910fps