Daryl,
I beg your pardon (and pjb also) as the load was wrong on my post. I have not my notes at hand when I wrote it.

I made the "smooth bore rifle" to shot an Indian hunting load of 6 drachms (164gn) and a light load of 3 drachms (82gn).
I do not know how Inkael FFFg powder compares to other powders as it is the only I can powder get but 164gn is a very strong load.
I proof the barrelled action as per British Proof house with double charge (330gn) of powder and 1.3 lead (but use 7# shot instead).
BTW: I also made a single barrelled howdah pistol on the same calibre proofed for 100gn BP.

Thank for your corrections.
Martin