Im pretty sure there is an exemption for "sporting firearms", hence the reason that over 1/2 inch guns can still be made, like 600 NE guns etc. Either way this is more of a fantasy discussion. So for the heck of it Im picturing something like a short (maybe 1 1/2-2") 58 cal case rimmed of course. Id like to use a round ball crimped into the case.

If forced to use cartridges in the 50 or less size, then Im struggling between 500 SW, 500 Linebaugh, 454C, or perhaps something like 45-70, maybe 458 Bushmaster, or similar.

To continue the Howdah theme, Im picturing using an Encore or Contender, even though Id prefer a double; but atleast the single barrel has the benefit of being thin in holster