Percussion cap

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

A primer is a small, disposable copper or brass cup, 1..2mm in diameter. In the cup is a precise, dried droplet of stable, but shock-senstive explosive, such as lead azide or potassium perchlorate[?].

A striker hits the outside of the cup, which bends, and the explosive is crushed on an anvil. The explosive explodes, igniting a secondary charge of gunpowder or other explosive.

Caps were originally manually placed on nipples on the outside of single-shot muzzle-loading weapons. Pulling the trigger released a hammer to crush the cap against the nipple.

Eventually, caps were incorporated into the rear of metallic cartridges. A small stamped anvil was added to the design, placed inside the cup to make the modern replaceable primer.

Corrosive primers use stable, long-lived explosives that generate acidic residues in a gun. They are popular with the military because they work.

Noncorrosive primers are somewhat less reliable when stored for many years, but far easier on gun. Most civilian ammunition uses noncorrosive primers.

History

The percussion cap replaced the posder 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 invention which made the percussion cap possible was patented by the Rev. A. J. Forsyth in 1807, and consisted of priming with a fulminating powder made of chlorate of potash, sulphur and charcoal, which exploded by concussion.

This invention was gradually developed, and 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 flint-lock to the percussion musket was easily accomplished by replacing the powder pan by a perforated nipple, and by replacing the cock or hammer which held the flint by a smaller hammer with a hollow to fit on 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 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, shot guns and pistols.

Caps are used in cartridges[?], grenades, and RPGs.