Too small as in bullet diameter or power? The 500-450 gun I posted above may be more powerful; but something loaded to 454 Casull or 460/500 SW is hard to argue with the power.

I agree 28 or 20 gauge would be more in keeping with traditional guns. Something in 577 (28 ga) might be perfect and IF in a period cartridge might even get the sporting gun exception to the greater than 50 caliber NFA issue here in the States.

Howdahs have a reputation for power; but how much power compared to modern nitro cartridges. I get the crushing power of a lead ball but how does that compare to penetration when it comes to stopping at tiger on your elephant range?

45/410 works from the "no legal question" position, if not the cool as big as your thumb look.