Before I shot anything out of that gun, I'd want a good gunsmith experienced with older shotguns to give it a thorough examination. The first thing you'd want to know is to what standards it was proofed. Then, I'd particularly want to confirm the barrel wall thicknesses, as it was not uncommon to hone pitted bores in the past and that gun looks old enough to have experienced black powder shells and corrosive primers. I'd also want to confirm that the chambers had not been lengthened. If they aren't marked as being 70 mm and they measure 2-3/4 inches, they have been. That would make confirming the barrel wall thicknesses all the more important.
Black powder rounds are not necessarily any lower in pressure than smokeless rounds, although the pressure curves can be quite different. The RST shells have worked well in my Damascus barreled guns, but I've only shot them in 10 gauge and 12 gauge, so I can't comment on their 16 gauge loads. I suspect you'll have trouble finding short 16 gauge shells, either RST or any other make in this market.
