Editing Cartridge (firearms) (section)

==Classification==
Cartridges are classified by some major characteristics. One classification is the location of the primer. Early cartridges began with the pinfire, then the rimfire, and finally the centerfire.

[[File:Simpified schematic of blowback mechanism with advanced primer ignition.png|thumb|Schematic of an Advanced Primer Blowback operation using a rebated rim cartridge (bottlenecked). The rebated rim allows the bolt to enter the chamber with the round to be fired in forward travel just before the cartridge in-battery position.]]

Another classification describes how cartridges are located in the chamber ([[headspace (firearms)|headspace]]). Rimmed cartridges are located with the rim near the cartridge head; the rim is also used to extract the cartridge from the chamber. Examples are the [[.22 long rifle]] and [[.303 British]]. In a rimless cartridge, the cartridge head diameter is about the same as or smaller than the body diameter. The head will have a groove so the cartridge can be extracted from the chamber. Locating the cartridge in the chamber is accomplished by other means. Some rimless cartridges are necked down, and they are positioned by the cartridge's shoulder. An example is the [[.30-06 Springfield]]. Pistol cartridges may be located by the end of the brass case. An example is the [[.45 ACP]]. A [[belted magnum|belted cartridge]] has a larger diameter band of thick metal near the head of the cartridge. An example is the [[.300 Weatherby Magnum]]. An extreme version of the rimless cartridge is the rebated case; guns employing [[Blowback (firearms)#Advanced primer ignition (API) blowback|advanced primer ignition]] need such a case because the case moves during firing (i.e., it is not located at a fixed position). An example is the [[Oerlikon 20 mm cannon|20mm×110RB]].

===Centerfire===
[[File:Fired rimfire and centerfire casings.jpg|thumb|Fired rimfire (left) and centerfire (right) cartridges. A rimfire [[firing pin]] produces a notch at the edge of the rim; a centerfire pin produces a divot in the center of the primer.]]
{{Main|Centerfire ammunition}}
A centerfire cartridge has a centrally located primer held within a recess in the case head. Most centerfire brass cases used worldwide for sporting ammunition use [[Boxer primer]]s. It is easy to remove and replace Boxer primers using standard reloading tools, facilitating reuse.

Some European- and Asian-manufactured military and sporting ammunition uses [[Berdan primer]]s. Removing the spent primer from (decapping) these cases requires the use of a special tool because the primer anvil (on which the primer compound is crushed) is an integral part of the case and the case, therefore, does not have a central hole through which a decapping tool can push the primer out from the inside, as is done with Boxer primers. In Berdan cases, the flash holes are located to the sides of the anvil. With the right tool and components, reloading Berdan-primed cases is perfectly feasible. However, Berdan primers are not readily available in the U.S.

===Rimfire===
[[File:Rim fire.svg|thumb|Schematic of a rimfire cartridge and its ignition]]
{{Main|Rimfire ammunition}}
Rimfire priming was a popular solution before centerfire priming was perfected. In a rimfire case, centrifugal force pushes a liquid priming compound into the internal recess of the folded rim as the manufacturer spins the case at a high rate and heats the spinning case to dry the priming compound mixture in place within the hollow cavity formed within the rim fold at the perimeter of the case interior.

In the mid to late 19th century, many rimfire cartridge designs existed. Today only a few, mostly for use in small-caliber guns, remain in general and widespread use. These include the .17 Mach II, .17 Hornady Magnum Rimfire (HMR), 5mm Remington Magnum (Rem Mag), .22 (BB, CB, Short, Long, Long Rifle), and .22 Winchester Magnum Rimfire (WMR).

Compared to modern centerfire cases used in the strongest types of modern guns, existing rimfire cartridge designs use loads that generate relatively low chamber pressures because of limitations of feasible gun design, as the rim has little or no lateral support from the gun. Such support would require very close tolerances in the design of the chamber, bolt, and firing pin. Because that is not cost-effective method, it is necessary to keep rimfire load pressure low enough so that the stress generated by chamber pressure would not push the case rim outward and cause the rim to expand significantly. Also, the wall of the folded rim must be both thin and ductile enough to easily deform, as necessary to allow the blow from the firing pin to crush the rim, thereby igniting the primer compound, and it must do so without rupturing the case. If the rim is too thick, it will be too resistant to deformation and if it is too hard, the rim will be to brittle and crack, rather than deform.<ref name="Williamson"/>

Modern centerfire cartridges are often loaded to {{convert|65000|psi|MPa|abbr=on}} maximum chamber pressure. Conversely, no commercialized rimfire has ever been loaded above {{convert|40000|psi|MPa|abbr=on}} maximum chamber pressure. However, with careful gun design and production, no fundamental reason exists that higher pressures could not be used. Despite the relative lower chamber pressure, modern rimfire magnums are commonly found in .17-caliber (4.5&nbsp;mm), .20-caliber (5mm), and .22-caliber (5.6&nbsp;mm) that can generate muzzle energies comparable to smaller caliber centerfire cartridges.{{Citation needed|date=May 2012}}

Today, [[.22 LR]] (.22 Long Rifle) accounts for the vast majority of all rimfire ammunition produced. Standard .22 LR rounds use an essentially pure lead bullet plated with a typical 95% copper, 5% zinc combination. These are offered in [[supersonic]] and subsonic types, as well as target shooting, plinking, and hunting versions. These cartridges are usually coated with hard wax for fouling control.

The .22 LR and related .22 rimfire cartridges use a [[heeled bullet]], where the external diameter of the case is the same as the diameter of the forward portion of the bullet and where the rearward portion of the bullet, which extends into the case, is necessarily smaller in diameter than the main body of the bullet.