I wholeheartedly agree that a howdah pistol needs to be a big-bore stopper, and like lancaster I would personally endorse the .577 minimum all day long.

But.....!

The only pistol that I know of (except the Pedersoli) that was actually called a howdah pistol by the maker, the Charles Lancaster 4-barrel pistol, was made in .380, .450 and .455, with .577 Snider as the upper limit!