Rimless

A rim is an external flange that is machined, cast, molded, stamped or pressed around 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 rimmed cartridge is the oldest of the types and 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 hold the cartridge at the proper depth in the chamber--this function is called \"headspacing\". Because the rimmed cartridge headspaces on the rim, the case length is of less importance than rimless cartridges. This allows some firearms chambered for similar rimmed cartridges to 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, where the rim helps hold the cartridge in position, and break-open single shot firearms. Rimmed cartridges generally don't work as well 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.

Examples of rimmed handgun cartridges include the .38 Special, .357 Magnum, .44 Magnum, etc. Rimmed rifle cartridge examples include the .22 Hornet, .303 British, and more.

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 and break-open single shot firearms, where headspacing off the case mouth prevents an aggressive crimp to hold the bullet in place against the heavy recoil of firing.

Read more at Wikipedia.org