I've got an itch and it needs scratching ....

I was dreaming of whether a current experienced double rifle maker could make a howdah pistol

An appropriately sized action. Under lever or side lever.

Action shaped or reshaped with a curved tang and curved trigger "plate".

Hammers.

Appropriate length barrels. Well regulated as well as a handgun can be.

Now what cartridge and chambering?

The .577 Snider was a 450 gr or 480 gr bullet. 70 grs of BP. A standard to aim by.

An obvious one is .45/70. .45, 450 yes, 70 grs BP.

A Nitro for Black in a modern gun is better.

But I think a .500 or a .577 is better.

The original .577 Snider a maybe.

What would be a modern choice?

Obviously flanged.

Maybe a shortened .500 NE case or a shortened .577 NE case?