Rimfire
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-1800s, 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). In some countries, rimfire cartridges up to at least .32-caliber are still used and even the .44 Henry (a circa 1860 cartridge) is still occasionally offered.
Compared to modern centerfire cases used in the strongest types of modern guns, existing rimfire cartridge designs use loads that generate relatively low pressure because of limitations of feasible gun design — the rim has little or no lateral support from the gun. Such support would require very close tolerances in design of the chamber, bolt, and firing pin. Because that is not cost-effective, it is necessary to keep rimfire load pressure low enough so that the stress generated by chamber pressure that would push the case rim outward cannot expand the rim significantly. Also, the wall of the folded rim must be thin and ductile enough to easily deform, as necessary to allow the blow from the firing pin to crush and thereby ignite the primer compound, and it must do so without rupturing, If it is too thick, it will be too resistant to deformation. If it is too hard, it will crack rather than deform. These two limitations — that the rim is self-supporting laterally and that the rim is thin and ductile enough to easily crush in response to the firing pin impact — limit rimfire pressures.
Modern centerfire cartridges are often loaded to about 65,000 psi (450,000 kPa) maximum chamber pressure. Conversely, no commercialized rimfire has ever been loaded above about 40,000 psi (280,000 kPa) maximum chamber pressure. However, with careful gun design and production, no fundamental reason exists higher pressures could not be used. Despite the relative pressure disadvantage, modern rimfire magnums in .17-caliber, 5mm, and .22-caliber generate muzzle energy comparable to smaller centerfire cartridges.
Today, .22 LR (Long Rifle) accounts for the vast majority of all rimfire ammunition used. 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, plinking, and hunting versions. These cartidges are usually coated with a hard wax for fouling control.
The .22 LR and related rimfire .22 cartridges also use a unique two-diameter bullet, where external diameter of the case is the same as 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. This is known as a heel-base bullet design.
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