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In firearm ballistics, the primer (/ˈpraɪmər/) is a component of pistol, rifle, and shotgun rounds. Early primers were simply the same black powder used to fire the weapon but poured into a pan or tube where it could be ignited by some external source of ignition such as a fuse or a spark. This external powder was connected though a tube in the barrel that led to the main charge. As powder won't burn when wet, this led to difficulty, or even the inability, to fire weapons in the rain.
Contents
- Priming methods
- Internal priming
- Rimfire
- Pinfire
- Peripheral primer
- Centerfire
- Teat fire
- Electric primed
- References
Modern primers are shock sensitive chemicals. In smaller weapons the primer is usually integrated into the rear of a cartridge. In larger weapons like cannon the primer is a separate component placed inside the barrel to the rear of the main propellant charge. Upon being struck with sufficient force generated by the firing pin or, typically in larger weapons, being electrically ignited, these primers react chemically to produce heat which ignites the main propellant charge and fires the projectile. Due to their small size these primers lack the ability to fire the projectile but still have enough energy to drive a bullet partway into the barrel (a dangerous condition called a squib load) and such a condition was instrumental in the death of Brandon Lee.
Priming methods
The first step to firing a firearm of any sort is igniting the propellant. The earliest firearms were cannons, which were simple closed tubes. There was a small aperture, the "touchhole", drilled in the closed end of the tube, leading to the main powder charge. This hole was filled with finely ground powder, which was then ignited with a hot ember or torch. With the advent of hand-held firearms, this became an undesirable way of firing a gun. Holding a burning stick while trying to pour a charge of black powder carefully down a barrel is dangerous, and trying to hold the gun with one hand while simultaneously aiming at the target and looking for the touchhole makes it very difficult to fire accurately.
Internal priming
Chemical primers, advanced metallurgy and manufacturing techniques all came together in the 19th century to create an entirely new class of firearm — the cartridge arm. Flintlock and caplock shooters had long carried their ammunition in paper cartridges, which served to hold a measured charge of powder and a bullet in one convenient package; the paper also served to seal the bullet in the bore. Still, the source of ignition was handled separately from the cartridge. With the advent of chemical primers, it was not long before several systems were invented with many different ways of combining bullet, powder, and primer into a single package which could be loaded quickly from the breech of the firearm. This greatly streamlined the reloading procedure and paved the way for semi- and fully automatic firearms.
This big leap forward came at a price. It introduced an extra component into each round – the cartridge case - which had to be removed before the gun could be reloaded. While a flintlock, for example, is immediately ready to be reloaded once it has been fired, adopting brass cartridge cases brought in the problems of extraction and ejection. The mechanism of a modern gun not only must load and fire the piece, but also must remove the spent case, which may require just as many moving parts. Probably most malfunctions involve this process, either through failure to extract a case properly from the chamber or by allowing it to jam the action. Nineteenth-century inventors were reluctant to accept this added complication and experimented with a variety of self-consuming cartridges before acknowledging that the advantages of brass cases far outweighed their one drawback.
The three systems of self-contained metallic cartridge ignition which have survived the test of time are the rimfire, the Berdan centerfire primer, and the Boxer centerfire primer.
Rimfire
Rimfire cartridges use a thin brass case with a hollow bulge, or rim, around the back end. This rim is filled during manufacture with an impact-sensitive primer. In the wet state, the primer is stable; a pellet of wet primer is placed in the shell and simply spun out to the full extremes of the rim. (For more on the exact process and one set of chemical compounds that have been used successfully, see U.S. Patent 1,880,235, a 1932 Remington Arms patent by James E. Burns.) In the dry state, the primer within the rim becomes impact-sensitive. When the rim is then crushed by the hammer or firing pin, the primer detonates and ignites the powder charge. Rimfire cartridges are single-use and normally cannot be reloaded. Also, since the rim must be thin enough to be easily crushed, the peak pressure possible in the case is limited by the strength of this thin rim. Rimfire cartridges originally were available in calibers up to 1" (100 caliber), the latter being the 1 Inch U.S. Gatling Gun Cartridge For The M1865, all but the small .22 caliber rounds eventually died out. The .22 Long Rifle, also fired in pistols, is the most popular recreational caliber today because it is inexpensive and quiet and has very low recoil.
While the rimfire priming method is limited due to the thin cases required, it has enjoyed a few resurgences recently. First was Winchester's .22 Magnum Rimfire, or .22 WMR, in the 1950s, followed in 1970 by Remington's short-lived 5mm Rimfire, based on Winchester's magnum case. In 2002 Hornady introduced a new .17 caliber cartridge based on the .22 WMR, the .17 HMR. The .17 HMR is essentially a .22 WMR cartridge necked down to accept a .17-caliber bullet, and is used as a flat-shooting, light-duty varmint round. The .17 HMR was followed a year later by Hornady's .17 Mach 2, or .17 HM2, which is based on a slightly lengthened and necked-down .22 Long Rifle cartridge. Both of the .17 caliber rimfires have had widespread support from firearms makers, and while the high-tech, high-velocity .17 caliber jacketed bullets make the .17 Rimfire cartridges quite a bit more expensive than the .22 caliber versions, they are excellent for shorter-range shooting and still far less expensive than comparable centerfire cartridges. In 2013, Winchester released the .17 Winchester Super Magnum, which utilizes a somewhat larger case than the .17 HMR, allowing for velocities approaching 3000fps with a 20gr bullet and making it the world's fastest rimfire round.
Pinfire
A pinfire firearm cartridge is an obsolete type of brass cartridge in which the priming compound is ignited by striking a small pin which protrudes radially from just above the base of the cartridge. Invented by Casimir Lefaucheaux in 1828 but not patented until 1835, it was one of the earliest practical designs of metallic cartridge. However, the protruding pin was vulnerable to damage, displacement and accidental ignition. Moreover, the pin had to be positioned carefully in a small notch when loading, making the pinfire's use in repeating or self-loading weapons impossible. The pinfire survives today only in a few very small blank cartridges designed as noisemakers and in novelty miniature guns.
Peripheral primer
This unique system, much like a refined combination of the pinfire and rimfire, uses a firing pin that strikes a ring of priming compound in the center of the cartridge as described in U.S. Patent 4,848,237. Despite its being successful, only experimental batches of the cartridge were made. The primary advantage is that it is struck from the side, which allows the operating system of the firearm to be moved forward allowing a more compact action. No commercial weapons used the system, however.
Centerfire
The identifying feature of centerfire ammunition is the primer -- a metal cup containing primary explosive inserted into a recess in the center of the base of the cartridge. The firearm firing pin crushes this explosive between the cup and an anvil to produce hot gas and a shower of incandescent particles to ignite the powder charge. Berdan and Boxer primers are used in centerfire cartridges; the primers differ in construction. American cartridges primarily use Boxer primers, and European cartridges use Berdan primers. Various priming mixtures have been used in different sized primers to effect prompt ignition of the powder charge. Particles with relatively high heat capacity are required to promptly ignite smokeless powder deterrent coatings. Some priming explosives decompose into incandescent solids or liquids. Inert ingredients may be heated into incandescent sparks when the explosive decomposes into gas. Cartridges for military use require stable priming formulations so war reserves of small-arms ammunition will dependably function after years of storage.
Teat-fire
The "Teat-fire" cartridges did not have a rim at the back like conventional cartridges, but were rounded at the rear, with a small "teat" that would protrude through a tiny opening in the rear of the cylinder.
Electric-primed
Some rapid-fire, and larger military gun and cannon rounds (such as the M50 20mm) utilize an internal electric primer that is activated by an externally provided electric charge, as opposed to a mechanical impact. The primer in turn ignites the primary propellant.