Velocity with the .577 Snider in a 10" barrel was ~920fps (standard BP rifle loads). A rapid second shot wouldn't be aimed--the smoke cloud was pretty dense.

When 50-70 brass wasn't obtainable, the standard dodge was to use .348 Winchester cases blown out to .50 caliber and trimmed to length. This might make a suitable howdah cartridge.

The .500 Linebaugh cartridge might be worth considering. Parent brass is a .348 Winchester, cut to 1.4", blown out, with the rim slightly reduced in diameter: for a howdah, you wouldn't need to reduce the rim. My old "plinking" load in this resulted in a 530 grain bullet at ~960fps. We also used to load what we jokingly called a "50 special": 500 grain wadcutter at 725 fps. (Velocities from a revolver with a 6.5" barrel.)