The percussion cap, introduced around 1830, was the crucial invention that enabled muzzle-loading firearms to fire reliably in any weather.
Before this development, firearms used flintlock ignition systems which 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 which 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 military firearms used 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.
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 most now use non-corrosive compounds such as lead styphnate.
The percussion cap replaced the flint, the steel "frizzen", and the powder pan of the flint-lock mechanism. It was only generally applied to the British military musket (the Brown Bess) in 1842, a quarter of a century after the invention of percussion powder and after an elaborate government test at Woolwich in 1834. The first percussion firearm produced for the US military was the M1819 Hall rifle.
The discovery of fulminates was made by Edward Charles Howard (1774-1816) in 1800. The invention that made the percussion cap possible using the recently discovered fulminates was patented by the Rev. Alexander John Forsyth of Belhelvie, Aberdeenshire, Scotland in 1807.
It consisted of priming with a fulminating powder made of fulminate of mercury, chlorate of potash, sulphur, and charcoal, which was exploded by concussion. It was an invention born of necessity: Rev. Forsyth had noticed that sitting birds would startle when smoke puffed from the powder pan of his flintlock shotgun, giving them sufficient warning to escape the shot. His invention of a fulminate-primed firing mechanism deprived the birds of their early warning system, both by avoiding the initial puff of smoke from the flintlock powder pan, as well as shortening the interval between the trigger pull and the shot leaving the muzzle.
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