Short barrel, roundball shooter, traditional style... Lyman Deerstalker (not the carbine because they are 1:24 twist). Another one would be a Tradition's PA Pellet. They come in flintlock and percussion. They also have a short then normal barrel.

Actually the Tradition's PA Pellet has a removable breech plug, will handle up to 150 grains of pellets, andare very reasonable.That is where I would look at.