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, the small percussion caps were difficult to handle under the stress of combat or while riding a horse. Several manufacturers developed alternate "auto-priming" systems. The "Maynard Tape Primer," for example, used a roll of paper "caps" much like today's toy cap pistols. 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.
A cartridge primer is a small copper or brass cup, containing a precise amount of stable but shock-sensitive explosive mixture, with ingredients such as lead azide or potassium perchlorate. Cartridge primers are 4 to 6 mm in diameter (standard sizes are 0.175 inches and 0.210 inches for handgun and rifle cartridges).
A striker hits the outside of the cup, which deforms, and the explosive is crushed on an anvil. The shock-sensitive chemical compound explodes, igniting a secondary charge of gunpowder or other explosive.
One kind of cartridge primer has an anvil which is part of the cartridge itself, and this type is called a Berdan primer. This type is the commonly used primer in European cartridges. A separate, small stamped anvil was later invented to replace the integral anvil of the cartridge, and this combination is the modern replaceable primer, also called a Boxer primer, and is commonly used in American cartridges. (Ironically, the Boxer primer is a European invention, and the Berdan primer is an American invention).
Corrosive primers use stable, long-lived explosives that generate corrosive residues in a gun, usually metallic oxides, which, when exposed to moisture, form hydroxides. They are popular in military applications because they work reliably under severe conditions, but more careful attention must be paid to cleaning the weapon after every use.
Noncorrosive primers are somewhat less reliable when stored for many years, but far easier on guns. Most civilian ammunition uses noncorrosive primers.
New on the market in the late 1990s are lead-free primers, which address concerns over the lead and other heavy-metal compounds found in older primers. The heavy metals, while small in quantity, are released in the form of a very fine soot. Some indoor firing ranges are moving to ban primers containing heavy metals due to their toxicity. Lead-free primers were originally less sensitive and had a greater moisture sensitivity and correspondingly shorter shelf life than normal noncorrosive primers. Since their introduction, lead-free primers have become equal in performance to lead-based primers, and are gradually gaining popularity.
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 Model 1833 Hall-North U.S. Breech-Loading Percussion Carbine.
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. Fulminate-primed guns were also less likely to misfire than flintlock guns. However, it was not until after Forsyth's patents expired that the conventional percussion cap system was developed.
Joshua Shaw, an English-born American, is sometimes credited with the development of the first metallic percussion cap in 1814, but his claim remains clouded with controversy as he did not patent the idea until 1822. Shaw’s percussion caps used a mixture of fulminate of mercury, chlorate of potash, and ground glass contained in a small metallic cup. Other possible claimants include Joseph Manton, Col. Peter Hawker, Francois Prelat, and most likely of all, Joseph Egg (nephew of Durs Egg).
This invention was gradually improved, and came to be used, first in a steel cap, and then in a copper cap, by various gunmakers and private individuals before coming into general military use nearly thirty years later.
The alteration of the military flintlock to the percussion musket was easily accomplished by replacing the powder pan and steel "frizzen" with a nipple, and by replacing the cock or hammer which held the flint by a smaller hammer formed with a hollow made to fit around the nipple when released by the trigger. On the nipple was placed the copper cap containing the detonating composition, now made of three parts of chlorate of potash, two of fulminate of mercury and one of powdered glass. The hollow in the hammer contained the fragments of the cap if it fragmented, reducing the risk of injury to the firer's eyes.
The detonating cap, thus invented and adopted, brought about the invention of the modern cartridge case, and rendered possible the general adoption of the breech-loading principle for all varieties of rifles, shotguns and pistols.