Percussion cap

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

Related Topics:
Firearm - Flints - Matches - Gunpowder

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A primer is a small, disposable copper or brass cup, 4 to 6mm in diameter (standard sizes are 0.175 inches and 0.210 inches for handgun and rifle cartridges). In the cup is a precise amount of stable, but shock-sensitive explosive mixture, with ingredients such as lead azide or potassium perchlorate.

Related Topics:
Explosive - Lead azide - Potassium perchlorate

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

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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. (See caplock mechanism.)

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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.

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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 with the many militaries because they work reliably under severe conditions.

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

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New on the market in the late 1990s are lead free primers, which address the concerns over the lead and other heavy metal compounds found in other primers. The heavy metals, while small in quantity, is 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.

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