Percussion cap

From Wikipedia, the free encyclopedia

The percussion cap or primer was the crucial invention that enabled firearms to fire in any weather. Before this development, firearms used igniters with flints or matches to set fire to a pan of gunpowder.

Pistol (left, fired) and shotgun (right) primers against an inch and mm scale

A 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. 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.

Caps were originally manually placed over hollow bronze nipples which also were seated into the rear end of muzzle-loaded weapon barrels. Pulling the trigger released a hammer to crush the cap against the nipple, causing a small explosion, which was guided down the hollow of the bronze nipple to ignite the gunpowder. (See caplock mechanism.)

Eventually, caps were incorporated into the rear of metallic cartridges, fitting flushly into a recess. One kind 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 military 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.

Large (top row) and small pistol primers. L-R fired, unfired, and inside view

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 other primers. The heavy metals, while small in quantity, are released in the form of a very fine soot, and many indoor firing ranges are moving to ban primers containing them for the potential health risk they pose. Lead-free primers were originally less sensitive (and thus less reliable) and had a much greater moisture sensitivity and correspondingly shorter shelf life than normal noncorrosive primers. Since their introduction, lead-free primers have improved to the point that they are nearly equal in performance to lead-based primers, and will likely become the norm in the near future.

[edit] History

The percussion cap replaced the powder pan of the wheel lock and flint-lock rifles. 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 discovery of fulminates was made by Edward Charles Howard (1774-1816) in 1800.[1] The invention that made the percussion cap possible using the recently discovered fulminates was patented by the Rev. A. J. Forsyth of Belhelvie, Aberdeenshire in 1807, and 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 delevoped.

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 with a perforated 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.

Caps are used in cartridges, grenades, Rocket propelled grenades, and rescue flares. Percussion caps have also been used as elements in the triggers of landmines.