DHE 16 Gauge SST Vent Rib:

[Bruce P Bruner]

My reasoning is twofold. First, I have a good supply of Black Powder & components and secondly, 7/8 ounce shot loads will be mild in terms of recoil for an 80+ year old stock. I am experienced in loading Black Powder in 12 gauge, 45-70, 45-75 and 45-90. The only disadvantage I have experienced is having to clean the breech chambers frequently when shooting all brass shot shells and the barrels do get very hot. I don't mind, I'm used to wiping between shots with my BP rifles. I think it's faster to clean a BP bore than with shooting smokeless, though I do stay on top with frequent cleaning so it's much easier at the end of the day. Water is still cheap and I'm pretty sure the shotgun doesn't care one way or the other.
I'm not opposed to using smokeless. I have a case of RST "Falcon Light" that should be a great load for the DH. I have a good supply of Super X 2-1/2 Dram Equivalent, 1oz. size 8 shot and Federal 2-1/2 Dr. Eq. 1oz. 7-1/2 shot. The stated velocity of the Federal is 1165fps. That's faster than any published Parker recommendations. I am not really comfortable using these in my 1938 Parker, but I'm all ears to hear your point of view.
I just checked the RST page and two flats of the "Falcon Light" plus shipping came to $405.00 ... My my!