For those without Austyn's book, this is what the .577 Howdah Pistol looks like. I WANT ONE!

Many large-bore Brit pistols were chambered for the .577 Revolver cartridge, which is a really short dumpy cartridge not much longer than a .22 rimfire. Unfortunately I don't have one in my collection, an omission I will have to rectify. Such pistols are very rare today. Its possible that a Howdah pistol in this configuration could be chambered for the more powerful short Snider cartridge, on the left in the photo below. The others are the standard-length Snider, the .577 Express, and the .577 Nitro Express.

I just love this calibre!

Regarding sleeving a small-bore shotgun action for a 'modern' Howdah pistol, I believe the .577 short Snider is still a realistic option. A single rifled .577 tube from any of the BP rifleworks like Dixie or Green River would provide the barrel sleeves, and the cases are available from Bertram. Lead projectiles from any .577 Minie mold would probably produce adequate short-range accuracy. Tons of fun, no doubt!