Everything in this thread can be applied to a single-barrel shotgun of 28-ga. or larger.
I don't shoot BP muzzleloaders anymore, but have a couple of single-barrel shotguns which I reload for, and I have BP muzzleloading adapters for 12-ga. which could substitute for cartridges if needed, as long as I had adequate caps or 209 sized primers (I have adapters for both).
But a well managed flinter can do amazing work.
I used one as a youngster, but no longer.
Is this what you use?
2 for a double barrel?