A Percussion Cap is an item required to load both the Percussion Rifle and Percussion Musket. In addition to a Percussion Cap, you're required to have a Musket Cartridge. A gun will have an empty durability meter when it is not loaded.If you select one of the two above guns on your hotbar and have both a Percussion Cap and a Musket Cartridge in your inventory, it will consume one of each, and begin filling the gun's durability meter. Once full, the gun will be ready to fire. After shooting, the gun's durability rating will go back to empty and another shot will need to be loaded. This whole process will only begin if you have both the Percussion Cap and Musket Cartridge in your inventory and will only continue while the gun to be reloaded is selected on your hotbar.