- Rim (firearms)
A rim is an external
flangethat 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. Nearly all modern ammunitionhas some type of rim, the major exception being caseless ammunition.
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 Specialcartridges 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 shotfirearms. 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.
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
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
crimpto hold the bullet in place against the heavy recoil of firing.
Examples of rimless handgun cartridges include the 9 mm Para,
.40 S&W, and .45 ACP. Rimless rifle examples include the .308 Winchester, .223 Remington, and .30-06 Springfield.
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.
.25 ACP, .32 ACP, and the .38 ACPare some handgun examples, while the .444 Marlinis a rifle cartridge that is semi-rimmed.
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 Eaglepistol. 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 941convertible 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.
.50 Beowulfalso uses a rebated rim design. This round is used in specialized AR-15upper 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.
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 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 reason for the lack of a definitive shoulder was that these old British cartridge cases were intended for firing corditecharges 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 H&H, 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 Jefferythat fit the same .512" bolt face used for the belted cases.
List of handgun cartridges
List of rifle cartridges
* cite book
last = Ackley
first = P.O.
title = Handbook for Shooters & Reloaders
edition = 12th Printing
series = vol I
origyear = 1962
year = 1927
publisher = Plaza Publishing
location = Salt Lake City, Utah
pages = 197-202
isbn = 978-9992948811
Wikimedia Foundation. 2010.
Look at other dictionaries:
Rim — may refer to: * Rim (coin), the sharp circular (or otherwise shaped) edge which surrounds the coin design * Rim (craters), the part of a crater that extends above the height of the local surface * Rim (firearms), a projection machined into the… … Wikipedia
Cartridge (firearms) — From left: .50 BMG, .300 Win Mag, .308 Winchester, 7.62×39mm, 5.56×45mm NATO, .22LR. A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm … Wikipedia
Blowback (firearms) — Blowback is a system of operation for self loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gases created by the ignition of the propellant charge. Several types of blowback… … Wikipedia
H & R Firearms — H R 1871, LLC Type Subsidiary Industry Firearms Founded 1871 Headquarters Ilion, N.Y. (manuf.) Madison, NC, (admin.) USA … Wikipedia
Headspace (firearms) — In firearms terms, the headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Used as a verb, headspace refers to the mechanism by which the… … Wikipedia
Extractor (firearms) — An extractor is a part in a firearm that holds onto the rim of the ammunition being fired. When the gun s action cycles, the extractor removes the spent casing from the firing chamber. Extractors can be found on bolt action, lever action, semi… … Wikipedia
Оружейная гильза — Запрос «Гильза» перенаправляется сюда; см. также другие значения. Чертеж гильзы винтовочного патрона .30 06 Springfield (США). Гильза с проточкой. Размеры в дюймах … Википедия
Antique guns — An Antique Firearm is, loosely speaking, a firearm designed and manufactured prior to the beginning of the 20th century the Boer War is often used as a cut off event, although the exact definition of what constitutes an Antique Firearm varies… … Wikipedia
Antique gun — An antique firearm is, loosely speaking, a firearm designed and manufactured prior to the beginning of the 20th century. The Boer War is often used as a cut off event, although the exact definition of what constitutes an antique firearm varies… … Wikipedia
Internal ballistics — Internal ballistics, a subfield of ballistics, is the study of a projectile s behavior from the time its propellant s igniter is initiated until it exits the gun barrel. The study of internal ballistics is important to designers and users of… … Wikipedia