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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 the place for the extractor to engage.
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.
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 do not 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.
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.
On a semi-rimmed case the rim projects slightly beyond the base of the case, though not as much as a rimmed cartridge. The tiny rim provides minimal interference feeding from a box magazine, while still providing enough surface to headspace on. Semi-rimmed cases are less common than the other types. 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.
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. One example of a rebated rim cartridge is the .50 Action Express (or .50 AE), commonly chambered in the 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 short-lived .41 Action Express (with the same rim diameter as 9×19mm Parabellum) used in the 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 Remington 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 .50 Beowulf 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.
Rebated rims were also a feature of the cartridges of automatic cannons derived from the 20-mm Becker, of which the best known belong to the Oerlikon family. These Advanced Primer Ignition blowback weapons feature straight-sided chambers that are longer than necessary to contain the case. The face of the bolt has the same diameter as the case and follows it into the chamber. This means that the extraction claw also has to fit within the chamber, and therefore the case has a rebated rim.
The 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 around 1910 with the .400-.375 Holland & Holland Magnum also known as the .375 Velopex. The addition of the belt allowed the cartridge to properly headspace, despite the relative lack of a definite shoulder. The reason for the lack of a definitive shoulder was that these old British cartridge cases were intended for firing cordite charges instead of modern smokeless powder. Cordite was extruded as spaghetti-like rods, so the cartridge cases had to be fairly cylindrical shaped to accommodate the cordite propellant rods. The belt was carried through on other cartridges derived from the .375 Velopex, like the .375 Holland & Holland Magnum of 1912, in some cases to prevent the higher-pressure magnum cartridge from accidentally being chambered in a gun with a chamber of similar size.
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 based on the .404 Jeffery that fit the same .512" bolt face used for the belted cases.