The Percussion Cap ignition system was developed in 1805 by the Reverend John Forsyth of Aberdeenshire. This firing mechanism is a great step in advancement from its predecessors because it does not use an exposed flashpan to begin the ignition process. Instead, it has a simple tube which leads straight into the gun barrel.

The key to this system is the explosive cap which is placed on top of the tube. The cap contains fulminate of mercury, a chemical compound which explodes when it is struck. This is the same stuff as is used in the paper or plastic caps in a child's cap gun. As illustrated above, when the cap is struck by the hammer, the flames from the exploding fulminate of mercury go down the tube, into the gun barrel, and ignite the powder inside the barrel to propel the bullet.

This firing mechanism provided a major advance in reliability, since the cap was almost certain to explode when struck. This mechanism is almost immune to dampness, though in a rainfall one must still be cautious to avoid getting water in the gun barrel or into the ignition system while loading the weapon. The percussion cap was the key to making reliable rotating-block guns (revolvers) which would fire reliably, and in the early 1800s several manufacturers began producing these multiple-shot sidearms in mass quantities. The percussion cap firing mechanism gave an individual soldier a weapon of precision and reliability which was used to devastating effect in the U.S. Civil War.