Delayed Ready sounds about right, and as for balancing using Deadly, that depends on the base stats of the gun and how it’s supposed to compare to other weaponry. Is it meant as something to start with, or as something to work for? Is it supposed to be relatively powerful in the setting, or more of a backup weapon for if there’s little better for choosing from? Is it something you’d let a level 1 have right as the game starts, or do you want a level 9 to seriously consider choosing it over the other options they have?
In one game that muzzleloader could be the gun you want them to have just for a couple combats before replacing it, and another it could be what you want the final boss to suffer death via, and a third anywhere between.