A centerfire cartridge is a cartridge with a primer located in the center of the cartridge case head. Unlike rimfire cartridges, the primer is a separate and replaceable component.
The centerfire cartridge has supplanted the rimfire in all but the smallest cartridge sizes. Except for low-powered .22 and .17 caliber cartridges, and a handful of antiques, all pistol, rifle, and shotgun ammunition used today is centerfire.
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Centerfire cartridges are more reliable for military purposes, because the thicker metal cartridge cases can withstand rougher handling without damage. The stronger base of a centerfire cartridge is able to withstand higher pressures than a thin rimfire cartridge. Higher pressures give a bullet higher velocity and greater energy. While centerfire cartridge cases require a complex and expensive manufacturing process, economies of scale are achieved through interchangeable primers for similar cartridges. The expensive individual cases can be reused after replacing the primer, gunpowder and projectile.
Handloading reuse is an advantage for rifles using obsolete or hard-to-find cartridges such as the 6.5x54 Mannlicher, or larger calibers such as the .458 Lott, for which ammunition can be expensive. The forward portion of some empty cases can be reformed for use as obsolete or wildcat cartridges with similar base configuration. Modern cartridges larger than .22 caliber are mostly centerfire. Actions suitable for larger caliber rimfire cartridges declined in popularity until the demand for them no longer exceeded manufacturing costs, and they became obsolete.
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.[1] Berdan and Boxer cartridge primers are both considered "centerfire" and are interchangeable; the same weapon can fire either Berdan- or Boxer-primed rounds if the cartridge dimensions are the same.
The two primer types are almost impossible to distinguish by looking at the loaded cartridge, though the two flash-holes can be seen inside a fired Berdan case and the larger single hole seen or felt inside a fired Boxer case. Berdan priming is less expensive to manufacture and is much more common in military-surplus ammunition made outside the United States.
Berdan primers are named after their American inventor, Hiram Berdan of New York who invented his first variation of the Berdan primer and patented it on March 20, 1866, in U.S. Patent 53,388. A small copper cylinder formed the shell of the cartridge, and the primer cap was pressed into a recess in the outside of the closed end of the cartridge opposite the bullet. In the end of the cartridge beneath the primer cap was a small vent-hole, as well as a small teat-like projection or point (later to be known as an anvil) fashioned from the case, such that the firing pin could crush the primer against the anvil and ignite the propellant. This system worked well, allowing the option of installing a cap just before use of the propellant-loaded cartridge as well as permitting reloading the cartridge for reuse.
Difficulties arose in practice because pressing in the cap from the outside tended to cause a swelling of the copper cartridge shell, preventing reliable seating of the cartridge in the chamber of the firearm. Berdan's solution was to change to brass shells, and to further modify the process of installing the primer cap into the cartridge, as noted in his second Berdan Primer patent of September 29, 1869, in U.S. Patent 82,587. Berdan primers have remained essentially the same functionally to the present day.
Berdan primers are similar to the caps used in the caplock system, being small metal cups with pressure-sensitive explosive in them. Modern Berdan primers are pressed into the "primer pocket" of a Berdan-type cartridge case, where they fit slightly below flush with the base of the case. Inside the primer pocket is a small bump, the "anvil", that rests against the center of the cup, and two small holes (one on either side of the anvil) that allow flash from the primer to reach the interior of the case. Berdan cases are reusable, although the process is rather involved. The used primer must be removed, usually by hydraulic pressure or a pincer or lever that pulls the primer out of the bottom. A new primer is carefully seated against the anvil, and then powder and a bullet are added.
Meanwhile, Edward M. Boxer, of the Royal Arsenal, Woolwich, England was working on a primer cap design for cartridges, patenting it in England on October 13, 1866, and subsequently received a U.S. patent for his design on June 29, 1869, in U.S. Patent 91,818.
Boxer primers are similar to Berdan primers with one major difference, the location of the anvil. In a Boxer primer, the anvil is a separate stirrup piece that sits inverted in the primer cup providing sufficient resistance to the impact of the firing pin as it indents the cup crushing against the pressure sensitive ignition compound. The primer pocket in the case head has a single flash-hole in its center. This positioning makes little or no difference to the performance of the round, but it makes fired primers vastly easier to remove for re-loading; the spent primer assembly is removed by pushing a thin metal rod through the flash hole from the open end of the case. A new primer, anvil included, is then pressed into the case using a reloading press or hand-tool. Since the primer cup, ignition compound and anvil are together as one piece, the anvil depth must be the correct match for the primer that is being inserted, so that the primer does not suffer excessive crushing potentially causing ignition (priming is done as the first step, after cleaning and resizing the empty case before the powder is added). Boxer priming is universal for US manufactured civilian factory ammunition, and contributes to the large number of shooters who reload their ammunition.
Boxer-primed ammunition is slightly more complex to manufacture, since the primer is in two parts with a drop of pressure sensitive compound, but automated machinery producing primers by the hundreds of millions has eliminated that as a practical problem and while the primer is one step more complex to make, the cartridge case is simpler to make, use and maintain. For users who buy brass cases for reloading, the initial cost is more than equalised by the decreased cost of firing reloaded rounds compared to buying factory loaded ammunition, which is often an excellent source of reusable brass (reloading ammunition can save significant costs compared with new factory rounds).
Early primers were manufactured with various dimensions and performance. Some standardization has occurred where economies of scale benefit ammunition manufacturers. Boxer primers for the United States market come in different sizes, based on the application. The types/sizes of primers are:
Examples of uses:
The primer size is based on the primer pocket of the cartridge, with standard types available in large or small diameters. The primer's explosive charge is based on the amount of ignition energy required by the cartridge design; a standard primer would be used for smaller charges or faster-burning powders, while a magnum primer would be used for the larger charges or slower-burning powders used with large cartridges or heavy charges. Rifle, large and magnum primers increase the ignition energy delivered to the powder, by supplying a hotter, stronger and/or longer-lasting flame. Pistol cartridges often are smaller than modern rifle cartridges, so they may need less primer flame than rifles require. A physical difference between pistol and rifle primers is the thickness of the primer's case; since pistol cartridges usually operate at lower pressure levels than rifles, their primer cups are thinner, softer, and easier to ignite, while rifle primers are thicker and stronger, requiring a harder impact from the firing pin.[2] (Despite the names pistol and rifle, the primer used depends on the cartridge, not the firearm; a few high-pressure pistol cartridges like the .221 Fireball and .454 Casull use rifle primers, while lower-pressure pistol and revolver cartridges like the .32 and .380 Autos, 9mm Luger, .38 Special, .357 Magnum, .44 Magnum and .45 ACP and traditional revolver cartridges like .32-20, .44-40 and .45 Colt, also used in lever action rifles, still would be loaded with pistol primers. Virtually all cartridges used solely in rifles do, however, use rifle primers.)
Primer manufacture and insertion is the most dangerous part of small arms ammunition production. Sensitive priming compounds have claimed many lives including the founder of the famous British Eley ammunition firm. Modern commercial operations use protective shielding between operators and manufacturing equipment.[3]
Early primers used the same mercury fulminate used in 19th century percussion caps. Black powder could be effectively ignited by hot mercury released upon decomposition. Disadvantages of mercuric primers became evident with smokeless powder loadings. Mercury fulminate slowly decomposed in storage until the remaining energy was insufficient for reliable ignition. Decreased ignition energy with age had not been recognized as a problem with black powder loadings because black powder could be ignited by as little energy as a static electricity discharge. Smokeless powder often required more thermal energy for ignition.[4] Misfires and hang fires became common as the remaining priming compound sputtered in old primers. A misfire would result if the priming compound either failed to react to the firing pin fall or extinguished prior to igniting the powder charge. A hang fire is a perceptible delay between the fall of the firing pin and discharge of the firearm. In extreme cases, the delay might be sufficient to be interpreted as a misfire, and the cartridge could fire as the action was being opened or the firearm pointed in an inappropriate direction.
Incandescent particles were found most effective for igniting smokeless powder after the primary explosive gasses had heated the powder grains. Artillery charges frequently included a smaller quantity of black powder to be ignited by the primer, so incandescent potassium carbonate would spread fire through the smokeless powder.[5] Potassium chlorate was added to priming mixtures so incandescent potassium chloride would have a similar effect in small arms cartridges.
Priming mixtures containing mercury fulminate leave metallic mercury in the bore and empty cartridge case after firing. The mercury was largely absorbed in the smokey fouling with black powder loads. Mercury coated the interior of brass cases with smokeless powder loads, and the higher pressures of smokeless powder charges forced the mercury into grain boundaries between brass crystals where it formed zinc and copper amalgams weakening the case so it became unsuitable for reloading. The United States Army discontinued use of mercuric priming mixtures in 1898 to allow arsenal reloading of fired cases during peacetime.[6] Frankford Arsenal primers used a mixture of potassium chlorate and antimony sulfide through the first half of the 20th century. These corrosive primers leave a residue of potassium chloride salt in the bore after a cartridge is fired. These hygroscopic salt crystals will hold moisture from a humid atmosphere and cause rusting.[7] These corrosive primers can cause serious damage to the gun unless the barrel and action are cleaned carefully after firing.
Civilian ammunition manufacturers began offering non-corrosive primers in the 1920s, but most military ammunition continued to use corrosive priming mixtures of established reliability.[8] The various proprietary priming formulations used by different manufacturers produced some significantly different ignition properties[9] until the United States issued military specifications for non-corrosive lead styphnate primers for 7.62x51mm NATO cartridge production. Most United States manufacturers adopted the military standard for their civilian production of Boxer primers.[10] Manufacturers subsequently offered more powerful magnum primers for uniform ignition of civilian long-range or big-game cartridges with significantly greater powder capacity than required for standard infantry weapons.
Other explosives used in primers include lead azide, potassium perchlorate, or diazodinitrophenol (DDNP). 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.
Military-surplus ammunition often uses inexpensive corrosive or slightly-corrosive Berdan primers because they work reliably under severe conditions, whereas modern Boxer primers are almost always non-corrosive and non-mercuric. Determination of corrosive or non-corrosive characteristics based on the primer type should consider these final headstamp dates of corrosive ammunition production:[11]
All shotgun shells (apart from specialized .22 cartridges which hold a very small amount of shot) are center fire. Most are made from plastic although some were and still are made of paper. Shotgun shells, like most centerfire cartridges, can be reloaded and used again. A shotgun shell consists of the brass hull, and the plastic or paper portion. The primer is set in the hull and ignites the powder. The explosion ejects the wad which contains the shot. Shotgun primers for use in cold weather may contain magnum explosive charges to counteract the slower ignition and burning of powders at low temperatures.