Rim (firearms)
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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. Nearly all modern ammunition has some type of rim, the major exception being caseless ammunition.
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[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] Rebated rim
Rebated rim cartridges have a rim that is significantly smaller in diameter than the base of the case, serving only for extraction. Functionally the same as a rimless case, the rebated rim provides some additional benefits when considered in conjunction with other cartridges. Perhaps the most famous rebated rim cartridge is the .50 Action Express (or .50 AE), chambered in the massive and expensive Desert Eagle pistol. In order to simplify production, and to decrease the cost of ownership, the .50 AE was designed with a rebated rim which matched the diameter of the rim of the .44 Magnum, which was the most common caliber used in the Desert Eagle. By using the same rim dimensions as the .44 Magnum, a Desert Eagle could be converted from .44 Magnum to .50 AE by merely changing the barrel and magazine. Other convertible cartridges, such as the .41 Action Express used in the short-lived Jericho 941 convertible pistol, would function in the same magazine, and thus required only a barrel change to change caliber. The recent (early 2000s) Winchester Short Magnum, Winchester Super Short Magnum, and Remington Ultra Magnum and Short Action Ultra Magnum families of rifle cartridges also feature rebated rims. In these cases, the rim was designed to fit bolt faces for existing magnum rifles, but the case was made wider to allow a greater powder capacity.
The 6.5 Grendel also uses a rebated rim design. This round is used in specialized AR-15 upper receivers, and the rim matches the size of the rim of the 7.62x39mm, allowing those parts to be used in the custom-built upper receivers.
[edit] Belted
The sole purpose of the "belt" on belted cases (often referred to as belted magnums) is to provide headspacing; the extractor groove is cut into the belt just as it is cut into the case head on a rimless case. The belt acts as a rim on what is essentially a rimless case. The design originated in England with the .375 H&H Magnum. The addition of the belt allowed the cartridge to properly headspace, despite the relative lack of a definite shoulder. The Belt was carried through on other cartridges derived from the .375 H&H, though most did not really need it. In the USA, the Belt became somewhat synonymous with "magnum" during the late 20th century. More recently, new "magnum" cartridges introduced in the USA have been rimless or used rebated rims that fit the same .512" bolt face used for the belted cases.
