The gun has both barrels opened to cyl. I can definitely use the RB as is and figure out a load for LB to change the pattern to a more IC or even M as desired.
I do have a pound or two of the nice shot which is harder than lead, but not as hard as tungsten.
So the general formula would be:

Heavier load at lower velocities with a harder, larger shot on a forgiving wad and slow burning powder. A buffered load may help keep the pellets together.

Thanks again for your help. It is greatly appreciated.