Rimless

A rim is a projection machined, cast, molded, stamped or pressed into the bottom of a firearms cartridge. The rim may serve a number of purposes, the most common being as place for the extractor to engage. more...

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Nearly all modern ammunition has some type of rim, the major exception being caseless ammunition.

Types

There are various types of firearms rims in use in modern ammunition. These types are rimmed, rimless, semi-rimmed, rebated rim, and belted. These categories describe the size of the rim in relation to the base of the case.

Rimmed

The oldest type, the rimmed cartridge has a rim that is significantly larger in diameter than the base of the cartridge. Rimmed cartridges use the rim to hold the cartridge in the chamber of the firearm, with the rim serving to prevent the cartridge from seating too deeply--this function is called \"headspacing\". Because the rimmed cartridge headspaces on the rim, the case length is of little consequence. This allows some firearms chambered for similar rimmed cartriges safely chamber and fire shorter cartridges, such as using .38 Special cartridges in a .357 Magnum revolver. Rimmed cartridges are well suited to certain types of actions, such as revolvers and break-open single shot firearms, but work poorly in firearms that feed from a box magazine.

Some types of rimmed cartridges, the rimfires, also use the rim to contain the priming compound used to ignite the cartridge.

Rimless

On a rimless case, the rim is the same diameter as the base of the case; it is known as an extractor groove. Since there is no rim projecting past the edge of the case, the cartridge must headspace on the case neck, for a straight walled case, or on the shoulder of the case for a bottlenecked case; the extractor groove serves only for extraction. The lack of a projecting rim makes rimless cases feed very smoothly from box magazines, and they are primarily used in firearms that feed from a box magazine. Rimless cases are not well suited to break-open and revolver actions, though they can be used with appropriate modifications, such as a spring-loaded extractor or, in a revolver, a moon clip. Rimless straight walled cases are problematic in applications such as magnum revolvers, where headspacing off the case mouth prevents an aggressive crimp to hold the bullet in place against the heavy recoil of firing.

Semi-rimmed

The rare semi-rimmed case was an attempt to make a rimmed cartridge that fed better out of a box magazine. The rim projects slightly beyond the base of the case, though not as much as a rimmed cardtridge. The .38 ACP, developed for early semiautomatic pistols, is the most common example of a semi-rimmed cartridge. The tiny rim provides minmal interference feeding from a box magazine, while still providing enough surface to headspace on, allowing the use of an aggressive crimp to hold the bullet in place. The .38 Super, a higher pressure loading of the old .38 ACP case, is notorious for being less accurate than rimless cases, and so most modern .38 Super handguns are chambered so that the cartridge headspaces off the case mouth, like a rimless case. IF the chamber is cut shallow, so the case headspaces off the mouth, the rim is used for extraction only; a standard chamber will use the rim for both headspacing and extraction.

Read more at Wikipedia.org