I am in agreement about not using Trail Boss in this pistol. Pressures are higher with TB than they are with black powder, as well as having a different pressure curve.
I would suggest the 20 bore loaded with round ball, at tiger repelling distances, even to 15 yards, would be slightly better at the job than the .577 Snider.
Better yet, would be a 16, 14 or 12 bore SxS pistol.
The reason being more than sufficient penetration, especially with hardened balls & the capability of heavier loads yet.