Rifling or smooth bore?
Sights or non sighted?

Two scenarios.
1. A modern Howdah designed to use in defence against a tiger on top of an elephant. The original purpose.
1a. Same but home self defence in a house against an intruder.
2. A modern Howdah to shoot targets in a sporting sense of competition.

A smooth bore with or without sights might do for the first one. I suggest a small front sight, or shotgun bead is the minimum.

Rifled and sights, at least a small front sight is probably better, for the second. Smooth bore might work.

The only reasons in my mind for going smooth bore would cost, using existing donor shotguns, and perhaps also shooting shot.

Otherwise if shooting ball, conicals, whatever, suitable rifling would be preferred.