Before this development, firearms used flintlock ignition systems that produced flint-on-steel sparks to ignite a pan of priming powder and thereby fire the gun's main powder charge (the flintlock mechanism replaced older ignition systems such as the matchlock and wheellock). Flintlocks were prone to misfire in wet weather, and many flintlock firearms were later converted to the more reliable percussion system.
The percussion cap is a small cylinder of copper or brass with one closed end. Inside the closed end is a small amount of a shock-sensitive explosive material such as fulminate of mercury. The percussion cap is placed over a hollow metal "nipple" at the rear end of the gun barrel. Pulling the trigger releases a hammer that strikes the percussion cap and ignites the explosive primer. The flame travels through the hollow nipple to ignite the main powder charge. Percussion caps were, and still are, made in small sizes for pistols and larger sizes for rifles and muskets.
While the metal percussion cap was the most popular and widely used type of primer, their small size made them difficult to handle under the stress of combat or while riding a horse. Accordingly, several manufacturers developed alternative, "auto-priming" systems. The "Maynard tape primer", for example, used a roll of paper "caps" much like today's toy cap gun. The Maynard tape primer was fitted to some firearms used in the mid-nineteenth century and a few saw brief use in the American Civil War. Other disc or pellet-type primers held a supply of tiny fulminate detonator discs in a small magazine. Cocking the hammer automatically advanced a disc into position. However, these automatic feed systems were difficult to make with the manufacturing systems in the early and mid-nineteenth century and generated more problems than they solved. They were quickly shelved in favor of a single percussion cap that, while awkward to handle in some conditions, could be carried in sufficient quantities to make up for occasionally dropping one while a jammed tape primer system reduced the rifle to an awkward club.
The first practical solution for the problem of handling percussion caps in battle was the Prussian 1841 (Dreyse needle gun), which used a long needle to penetrate a paper cartridge filled with black powder and strike the percussion cap that was fastened to the base of the bullet. While it had a number of problems, it was widely used by the Prussians and other German states in the mid-nineteenth century and was a major factor in the 1866 Austro-Prussian War.
In the 1850s, the percussion cap was first integrated into a metallic cartridge, which contained the bullet, powder charge and primer. By the late 1860s, breech-loading metallic cartridges had made the percussion cap system obsolete. Today, reproduction percussion firearms are popular for recreational shooters and percussion caps are still available (though some modern muzzleloaders use shotshell primers instead of caps). Most percussion caps now use non-corrosive compounds such as lead styphnate.