Yes I agree 54 is small for a Howdah; but thats the largest commercially available for a rifled B/P pistol. Pedesoli also makes a 20ga percussion (not sure if it smooth or rifled).

The 45/410 is even smaller but at least is a cartridge version. I suspect the design is based on a 410 shotgun action, so Id be hesitant to load it too hot OTOH 45 Colt even Cowboy loads killed a lot of villains on the movies.... I do wish they made it in something like 50 Special. Im not sure any SxS shotgun action will 500S&W. I have a 10" Encore in 500S&W its definitely too light to "want" to shoot for anything short of a charging dinosaur.