Finally we have arrived at my own personal concept of a modern howdah pistol build, so its time to add my two cents worth. Except mine would definitely need rifling!

Several years ago I went down this rabbit-hole with a view to making a version of the SxS 16-bore hammergun that could be used in our Big Game Rifle competition for the Howdah Pistol event.
A couple of important pre-requisites were NO TOP-LEVER, and percussion-style fences. The problem with using a vintage 12-bore hammer-gun as a donor is that back-action locks cannot be contoured down into the pistol grip, as mentioned above: they are all too straight.
Unfortunately as the SxS shotgun evolved, the top-lever was universally adopted BEFORE the back-action lock was replaced by the bar-action lock, so finding a donor hammer-gun that had bar-action locks with either a Jones underlever or a side-lever was like looking for rocking-horse sh!t!

Anyway, long story short, I finally found one and made the acquisition. According to my gunsmith, I can do everything required except shortening the barrels. That requires changing the firearm class from A (shotgun) to H (handgun) in the NT, and this can only be done legally by a licenced armourer or gunsmith. Difficulties are predicted if I want it to remain an antique (pre-1900 manufactured action) as our Firearms Act technically allows. ...and I do!

The next problem is finding a donor barrel that I could get two short rifled 16-bore sleeves out of. New or vintage, don't really care, but the rifling would need to be "pukka", meaning wide grooves and relatively tall, narrow lands for round-ball, with a twist-rate in the order of 30 to 60 inches. In Australia! More rocking-horse sh!t needing to be sought!

Anyway, the donor hardware is in my gunsafe, so Step 1 is done. I have had the brass 16-bore cases forever, but not yet cut to 2 inches for the 16-bore short cartridge (to further circumvent Police accusations of a cut-down shotgun).
Anyway, this is already further than I manage to get on most of my conceptual projects!

Rest assured I'll post progress here if this project ever bubbles to the surface!