In the 12 gauge, I routinely load just under 18 gr/Clays with 7/8 oz of shot. Gray Claybuster was, STS hulls, Win 209 primers. Velocity is about 1240 fps, wonderfully soft recoil.
In your Superposed, I like to load 3/4 oz of shot, 15.5 gr/Unique, Claybuster 34 wad, AA hull, Win 209 primer. About 1200 fps. Very light recoil. If this load will not switch the to the second barrel on firing the first, a number of good gunsmiths can adjust it.
