When I first shot PRB in a smooth bore there was no info available to me as to how it was being done. Without giving it much thought, since I was shooting a double-barrel shotgun, I loaded just as I did for shot, hard card, cushion wad, PRB, and had excellent results. I've continued to do it that way most of the time. I also shoot a fair amount with hard card, cushion wad, bare ball, thin OS card. That very nearly gives me the same accuracy, so that makes me think the card+cushion wad is doing something good.
Here are the very first two targets I ever shot with PRB in a smooth bore, from a 20-gauge flint double, front bead only.
You can see why I have kept doing it that way. Using my single-barrel flint smoothbore loaded the same way, no rear sight, I've shot 3 1/2" groups at 75 yards, so I expect I'll keep doing it.
I don't know why it works so well for me. I suspect it has to do with getting a better gas seal, but don't know how to prove that.
I'll have to confess I've been so pleased doing it this way I've never given simply a PRB over powder a fair test.
A happy side benefit for me is that the lubed cushion wad helps keep the bore clean, I rarely have to wipe it in a hunting situation.
Spence