There is the need to use sufficient overpowder cards / wads so there is a firewall to prevent the fire and gas pressure from bleeding up into the shot charge and wrecking it.
I am not an authority on the subject at all...I have done a lot of study and made several testing trips to the range with .56cal, .62cal, and .12ga smoothbores, and found the best and simplest choice for me was to use OxYoke prelubed wonder wads.
I put 2 or 3 of them over the powder as a firewall, and to provide natural lube 1000 in the bore to minimize fouling, and to serve as a cushion wad for the shot cup to sit on.
In the .56 & .62 I use home made paper shot
[url] cups...in[/url] the .12ga I take a modern .12ga shotshell wad and cut off the skirt just below the floor of the shot cup itself so the shot cup sits down on the stack of wonder wads. In all cases I then use the normal .030" over shot card to hold the shot in place.
I know that a pure traditionalist would not use the plastic shot cup as I did, and would experiment with different wad, powder, and shot combinations instead until one was found that produced the best pattern out of the cyclinder bore.
But since my .12ga Navy Arms SxS is a percussion to begin with and not a flintlock, I decided to try the plastic shot cups for Doves a couple times last year and they did fine. Time permitting, I may go back and see what I can do without the plastic shot cup.