The only .577 Howdah pistol I'm familiar with (passing familiarity, that is) is in .577 Snider. That would be a 480 to 500gr. bullet and 85gr. rifle powder.(BP)
: Some years ago, Guns and Ammo published an aritcle on a couple Howdahs.
: The side by side was a 12 bore and the .577 was a 4-barrel.
: Recently, here, there was a posting with side by side .577 Snider pictured.