Tried shot capsules once!
Never attempted what your talking about. Never really thought about it. I tried plastic shot capsules once in my Dragoon. Got some .45 cal shot capsules and filled them with #12 shot. I loaded them in the chamber with a ML ball starter as they are easy to break with the ram!
Put a modest load of BP in the chamber and gently pressed the capsule into place with the wooded ball starter. I loaded only one chamber as I felt that recoil would mess up another or make it move forward and jamb things up.
Firing that shot cap with the #12 in it was marginal as best. I did hit a paper plate at about 6 ft., but after that it opened up to where the shot was so scattered I could not hit a flock of barns if they flew in front of me!
I suppose a wad or card atop the powder, then the shot, then another thin wad then the ball atop that may work, but I'm just speculating here and have no evidence that it would work.
I hope you can find what you seek. Just be careful and go slowly with this project is all I can say. I sure would not like to hear about an obstructed barrel and a bad outcome from this!!
Wade
Wade