A percussion cap is a small metal cap that contains a sensitive explosive compound. It is used in firearms, particularly in muzzle-loading guns, to ignite the main charge of gunpowder. When struck by the hammer of the firearm, the cap detonates, creating a spark that ignites the powder and fires the bullet. Originally developed in the early 19th century, percussion caps replaced the older flintlock ignition system. They provided a more reliable and weather-resistant means of firing a gun. The invention of the percussion cap significantly improved the efficiency and safety of firearms, paving the way for modern ammunition systems like centerfire and rimfire cartridges.