Editing Cartridge (firearms) (section)

===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.