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Centerfire cartridges have supplanted the rimfire variety in all but the smallest cartridge sizes. With the exception of some .22 and .17 caliber pistol and rifle cartridges, small-bore shotgun cartridges (intended for pest-control), and a handful of antique, mostly obsolete cartridges, almost all pistol, rifle, and shotgun ammunition used today is centerfire.
Centerfire cartridges are more reliable for military purposes, because the thicker metal cartridge cases can withstand rougher handling without damage. Centerfire cartridges are safer to handle; because explosive priming compound in a protruding rim is more likely to be detonated by impact if a rimfire cartridge is dropped or pinched. The stronger base of a centerfire cartridge protects the central primer from side impact, and 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, explosive handling is simplified by avoiding the spinning process required to uniformly distribute priming explosive into the rim because of uncertainty about which angular segment of a rimfire cartridge rim will be struck by the firing pin. Larger caliber rimfire cartridges require greater volumes of priming explosive than centerfire cartridges, and the required volume may cause an undesirably high pressure during ignition. Reducing the amount of priming explosive would reduce the reliability of rimfire cartridge ignition, and increase the probability of misfire or dud cartridges.
Economies of scale are achieved through interchangeable primers for a wide variety of centerfire cartridge calibers. The expensive individual brass cases can be reused after replacing the primer, gunpowder and projectile. Handloading reuse is an advantage for rifles using obsolete or hard-to-find centerfire 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 which is a metal cup containing a 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 cartridge primers are both considered "centerfire" and are not interchangeable at the primer level; however, the same weapon can fire either Berdan- or Boxer-primed cartridges if the overall 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, 1868, 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, Colonel Edward Mounier 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 and crushes 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, as a single, centered rod pushed through the flash hole from the open end of the case will eject the two-piece primer from the primer cup. A new primer, anvil included, is then pressed into the case using a reloading press or hand-tool. Boxer priming is universal for US-manufactured civilian factory ammunition.
Boxer-primed ammunition is slightly more complex to manufacture, since the primer is in two parts in addition to the 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 reload.
Boxer primer sizes
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:
- 0.175" (4.45 mm) diameter small pistol primers, and a thicker or stronger metal cup small rifle version for use with higher pressure loadings in weapons with heavy firing pin impact.
- 0.209" (5.31 mm) diameter primers for shotgun shells and modern inline muzzleloaders, using a Boxer-type primer factory-assembled inside a tapered, flanged brass cup.
- 0.210" (5.33 mm) diameter large rifle primers, and a thinner or softer metal cup large pistol version for use with lower pressure loadings in weapons with light firing pin impact. Large rifle primers are also 0.008" taller than large pistol primers. 
- 0.315" (8.00 mm) diameter .50 BMG primers, used for the .50 Browning Machine Gun cartridge and derivatives
Examples of uses:
- .38 Special, small pistol standard
- .357 Magnum, small pistol magnum
- .45 ACP, large pistol standard
- .223 Remington, small rifle standard
- .308 Winchester, large rifle standard
- .270 WSM, large rifle magnum
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. (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.)
All modern shotgun shells (excluding specialized rimfire .22 and 9 mm "snake loads" or birdshot cartridges) are centerfire. They use a large, specific shotgun primer that is based on the Boxer system, in that the primer contains the anvil against which the primary explosive is compressed by the firing pin and deformation of the primer cup.
Shotgun primers are also used as a replacement to the percussion cap ignition system in some modern black-powder firearms.
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.
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. 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. Potassium chlorate was added to mercury fulminate 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. Frankford Arsenal FA-70 primers used potassium chlorate as an oxidizer for lead(II) thiocyanate, to increase the sensitivity of potassium chlorate, and antimony trisulfide, as an abrasive, with minor amounts of trinitrotoluene. 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. 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. The various proprietary priming formulations used by different manufacturers produced some significantly different ignition properties until the United States issued military specifications for non-corrosive primers for 7.62x51mm NATO cartridge production. The PA-101 primers developed at Picatinny Arsenal used about 50% lead styphnate with lesser amounts of barium nitrate, antimony trisulfide, powdered aluminum and tetrazene. Most United States manufacturers adopted the PA-101 military standard for their civilian production of Boxer primers. 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 can include lead azide, potassium perchlorate, or diazodinitrophenol (DDNP). New on the market in the late 1990s are lead-free primers(see green bullet), to 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 better in their performance compared to early lead free primers, as reported by AccurateShooter.com in October 2011. Tests comparing lead-free primers to lead-based primers conducted by the US Department of Defense (approx 2006), exposed some significant differences in accuracy between the two primers when used in 7.62x51. In these tests, lead-free primers were proven to be not as accurate as lead-based primers. The lead-free primers exhibited poor performance as far as peak blast pressure, which consequently leads to poor ignition. Popularity is still minimal, as accuracy is paramount. Most lead-free primers are sourced through Russia (MUrom?)or South Korea (PMC).
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:
- .45 ACP: FA 54, FCC 53, RA 52, TW 53, WCC 52, WRA 54
- .30-06 Springfield: FA 56, LC 52, RA 51, SL 52, TW 52, WCC 51, WRA 54
- FN 57
For more detailed information on identifying USGI corrosive and non-corrosive ammunition based on cartridge headstamp, see Corrosive Primer Redux by M.E. Podany, ALGC. This article refers to The American Rifleman, "Beginners Digest: Nonmercuric, Noncorrosive Primers", pp. 34–36, January 1961.
- Treadwell, T.J. (1873). Metallic Cartridges, (Regulation and Experimental,) as Manufactured and Tested at the Frankford Arsenal, Philadelphia, PA. Washington, DC: United States Government Printing Office. p. 9.
- Davis, William C., Jr. Handloading (1981) National Rifle Association p.65
- Sporting Arms and Ammunition Manufacturers' Institute
- "FAQ". Retrieved 27 March 2014.
- Calhoon, James (October 1995). "Primers and Pressure". Varmint Hunter.
- Lyman Ideal Hand Book No. 36. Lyman Gun Sight Corporation (1949) p.45.
- Sharpe, Philip B. Complete Guide To Handloading (1953) Funk & Wagnalls p.51
- "PowerLabs Fulminate Explosives Synthesis". PowerLabs. Retrieved 2012-06-07.
- Lyman Ideal Hand Book No. 36 Lyman Gun Sight Corporation (1949) p.49
- Fairfield, A.P., CDR, USN Naval Ordnance (1921) Lord Baltimore Press pp.48-49
- Davis, William C., Jr. Handloading (1981) National Rifle Association p.20
- Lake, E.R. & Drexelius, V.W. Percussion Primer Design Requirements (1976) McDonnell-Douglas
- Sharpe, Philip B. Complete Guide To Handloading (1953) Funk & Wagnalls p.60
- Davis, William C., Jr. Handloading (1981) National Rifle Association p.21
- Landis, Charles S. (1947). Twenty-Two Caliber Varmint Rifles. Harrisburg, Pennsylvania: Small-Arms Technical Publishing Company. p. 440.
- Sharpe, Philip B. Complete Guide To Handloading (1953) Funk & Wagnalls p.239
- Davis, William C., Jr. Handloading (1981) National Rifle Association pp.21-22
- Davis, William C., Jr. Handloading (1981) National Rifle Association p.12