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Rim (firearms)
Rim (firearms)
Rim (firearms)
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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. Thus, rimmed cartridges are sometimes called "flanged" cartridges. Almost all cartridges feature an extractor or headspacing rim, in spite of the fact that some cartridges are known as "rimless cartridges". The rim may serve a number of purposes, including providing a lip for the extractor to engage, and sometimes serving to headspace the cartridge.

Types

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

Rimmed .357 Magnum revolver ammunition

Rimmed

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Main article: List of rimmed cartridges

The rimmed cartridge, sometimes called flanged 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 (usually straight sided) 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 with rimless cartridges. Rimmed cartridges with straight walls, such as shotgun shells, allow various lengths of the same cartridge to be chambered in the firearm; if the round protrudes into the rifling of the barrel past the length of the chamber, the rifling can act as a fluted chamber to ease extraction.

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, as these are the same diameter despite the nomenclature. Rimmed cartridges are well suited for certain types of actions, such as revolvers and break-action firearms, where the rim helps hold the cartridge in position.

Rimmed vs rimless cartridges

Rimmed cartridges generally do not work quite as well in firearms that feed from a box magazine, since the magazine must be carefully loaded so that the rim from each successive case is loaded ahead of the round beneath it, so the round will not snag on the rim of the cartridge below it as the bolt strips it out of the magazine. However, box magazine firearms firing rimmed cases have seen extensive use; the famous Lee–Enfield rifle used by the UK, and the Russian Mosin-Nagant rifle which uses an interrupter to help prevent the snags, were both used from before World War I until after World War II. Semi-automatic handguns have been chambered in rimmed cartridges as well, for example a LAR Grizzly or Desert Eagle in .357 Magnum or .44 Magnum. The .22 Long Rifle is extremely popular in semi-automatics.

Rimmed cartridges work with belt-fed machine guns that use a two stage pull out – push through feeding operation, notably the Maxim gun, Vickers, M1919 Browning and M2HB. Push through links are possible with rimmed cartridges using specially designed belt links, but not as reliable as using push through links with rimless ammunition.

Rimfire cartridges also use the rim to contain the priming compound to ignite the cartridge instead of a centrally mounted primer, as is commonly used in centerfire cartridges.[1]

Under the metric cartridge designation system, a capitalized "R" added at the end of the designation denotes a rimmed cartridge. For example, "7.62×54mmR" is a rimmed cartridge, while "7.62×51mm" is a rimless cartridge. Under imperial or US customary designations, there is typically no distinction between rimmed and unrimmed cartridges, unless one is referring to a rimmed version of a cartridge which is typically rimless, such as the .45 Auto Rim, a special rimmed version of the .45 ACP ("Automatic Colt Pistol" aka ".45 Auto"), intended for use in M1917 service revolvers.

Examples of rimmed handgun cartridges include the .38 Special, .357 Magnum, .44 Special, .44 Magnum, .45 Schofield, and .45 Colt. Rimmed rifle cartridge examples include the .22 Hornet, .30-30 Winchester, 7.62×54mmR, .303 British, 8×50mmR Lebel, and .45-70 Government.[2]

Rimless 9mm Parabellum pistol cartridges

Rimless

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As early as 1867 Joseph Whitworth patented a round which "has a solid metal back without any projecting flange, as heretofore, the ring groove already mentioned being a substitute for it".[3] Even though some rifles were designed for rimless cartridges by Bethel Burton, Eduard Rubin and others, the first adoption did not come until 1888 with its Patrone 88.

The rim on a "rimless" case is almost or exactly the same diameter as the base of the case. A recess formed between the rim and the body of the cartridge is known as an extractor groove, allowing the case to be grasped by an extractor after being fired. Since there is no rim projecting past the edge of the case, the cartridge must headspace on the case mouth, for a straight walled case, or on the case's shoulder for a bottlenecked case (although a bottlenecked case can headspace on the case mouth, depending on the cartridge); the extractor groove serves only for extraction. The lack of a projecting rim makes rimless cases feed very smoothly from box magazines, drum magazines, and belts. Rimless cases are not well suited to break-open and revolver actions, though in break-action firearms they can be used with appropriate modifications, such as a spring-loaded extractor/ejector or, in a revolver, a half or full moon clip (for example, the Colt or Smith & Wesson M1917 revolvers in .45 ACP).[4]

Since a straight-walled rimless cartridge is designed to headspace off of the case mouth, this prevents the ammunition loader or manufacturer from using a heavy crimp, which is a ring pinched or "crimped" into the cartridge case, designed to lock the bullet securely in place until fired. Crimping affects the overall length of the cartridge, and thus cannot be used on cartridges which headspace on the case mouth. This can be a problem for magnum revolvers or rifles which simultaneously chamber more than one round of ammunition, as the recoil from the firing successive rounds can loosen the bullets in the remaining cartridges, and cause their bullet seating depth to change, which can have a serious effect on accuracy, or could, in the case of a revolver, cause a bullet to protrude sufficiently from the front of the cylinder to obstruct the revolving of the cylinder thus jamming the gun from firing additional rounds. This is not an issue for break-action single shot firearms, since there is no cycling action to jam, although it could potentially cause accuracy (but not jamming) problems in double barreled rifles, double barreled shotguns, or combination guns, provided they have more than one rifle barrel. Some combination guns, such as "drillings" or "vierlings" are made with three or four rifle barrels, without any shotgun barrels.

Examples of rimless handgun cartridges include the .380 ACP, 9mm Parabellum, .357 SIG, .38 Super, .40 S&W, 10mm Auto, .45 GAP, .45 ACP, .50 AE and .50 GI. Rimless rifle examples include the .223 Remington, 6.5×52mm Carcano, 6.5×54mm Mannlicher–Schönauer, 6.5×55mm Swedish, .308 Winchester, .30-06 Springfield, 7.65×53mm and 7.92×57mm Mauser.[4]

Semi-rimmed

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Semi-rimmed .32 ACP pistol cartridges

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.[4]

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.[5]

Examples of semi-rimmed handgun cartridges are .25 ACP, .32 ACP, 8×22mm Nambu, .38 ACP, .38 Super, and 9mm Browning Long. Semi-rimmed rifle examples include the .220 Swift, .280 Ross, 6.5×50mm Arisaka, .308 Marlin Express, .338 Marlin Express, and .444 Marlin.[4]

Rebated

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Main article: List of rebated-rim cartridges
The .50 GI (left) has a rebated rim the same size as the rimless .45 ACP (right)
  • A: Normal rim
  • B: Rebated rim

Rebated cartridges have a rim that is significantly smaller in diameter than the base of the cartridge case, serving only for extraction. Functionally the same as a rimless case, the rebated rim allows a gun to be easily converted to fire a larger-than-normal cartridge, as most of a firearm's loading and extraction mechanism does not need to be altered as long as the rim size is preserved.[4] Another advantage with rebated-rim cartridges, mostly with straight-walled examples allow the usage of virtually any lengths of cartridge of the same caliber.

An example of a rebated-rim cartridge is the .50 Action Express, 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, the most common caliber used in the Desert Eagle pistol. By using the same rim dimensions, a Desert Eagle could be converted from the .44 Magnum to the .50 Action Express 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 and Uzi submachine gun and carbine, would function in the same magazine, and thus required only a barrel change for a different caliber.[6][7]

The .440 Cor-Bon is another cartridge with a rebated rim the same diameter as the .44 Magnum. The FN 5.7×28mm (used in the FN Five-seveN semi-automatic handgun and FN P90 personal defense weapon) is a well-known cartridge with a slightly rebated rim, but the reason for the choice is not clear, as there is no other cartridge it is known to be compatible with.

The recent (early 2000s) Winchester Short Magnum, Winchester Super Short Magnum, Remington Ultra Magnum and Remington Short Action Ultra Magnum families of rifle cartridges also featured rims that are rebated. All of these cases were based on the .404 Jeffery with the rim reduced from .543 inches to fit the .532 inch bolt face for existing magnum rifles.[8][7]

The only known shotgun shells using rebated rims is the Roper repeating shotgun[9] and the 12 Gauge RAS12, specially made for the RAS-12 semi-automatic shotgun.[10]

The .50 Beowulf rifle cartridge 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 7.62×39mm, allowing those parts to be used in the custom-built upper receivers.[8]

Other rebated rifle cartridges include the .350 Legend, .400 Legend, .450 Bushmaster, .458 SOCOM, .500 Jeffery, and .375 SWISS P.

Rebated cartridges are used for a different reason on automatic cannons derived from the 20-mm Becker, of which the best known belong to the Oerlikon family. These "advanced primer ignition" (API) blowback weapons feature straight-sided chambers which 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's rim has been rebated.

An unusual example of rebated-rim autocannon rounds has been used in the T168 autocannon prototype, the 3%1×120mmRB T268 which was designed for reverse loading of the rounds into the chamber. A telescopic example existed, the 23×260mm round used in the Rikhter R-23 autocannon that operated in a similar way.

Belted

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Main article: Belted magnum
Belted .375 H&H Magnum (left), .338 Winchester Magnum (right), US quarter (24 mm) for scale

The original purpose of the "belt" on belted cases (often referred to as belted magnums) was 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 Belted Nitro Express (also known as the .375/.400 Holland & Holland, and .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 belted .375 Holland & Holland Magnum of 1912, in some cases to allow the cartridge to function in bolt-action rifles (the original .375 H&H Magnum was a rimmed case for use in double-barreled rifles), or to prevent the higher-pressure magnum cartridge from accidentally being chambered in a gun with a chamber of similar size.[11]

Examples of belted handgun cartridges include the .40 BSA Auto Pistol and .40 G&A Magnum. Belted rifle / machine gun examples include the .224 Weatherby Magnum, .300 Winchester Magnum, .375 H&H Magnum, .450 Marlin, .458 Lott, 13×64mmB and .55 Boys.

In the United States, the belt became somewhat synonymous with "magnum" during the late 20th century. More recently, new "magnum" cartridges introduced in the United States have been rimless or used rebated rims based on the .404 Jeffery that fit the same .512" bolt face used for the belted cases.[12]

See also

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Rim (firearms)

View on Grokipedia
from Grokipedia
In firearms, the rim refers to an external machined, cast, or otherwise formed around the base of a metallic cartridge case, which protrudes beyond the of the case body to facilitate extraction from the chamber and to provide a surface for headspacing in certain designs. Cartridges are categorized by rim configuration into several types: rimmed, where the rim significantly extends outward and often includes a groove for the extractor; semi-rimmed, featuring a rim only slightly larger than the case body with a visible extractor groove; and rimless, in which the rim matches the case body, relying on an extractor groove alone for handling. These designs influence compatibility with various firearms, from revolvers and bolt-action to semi-automatic and automatic weapons. Rimmed cartridges, among the earliest metallic designs developed in the 19th century, are particularly suited for revolvers, lever-action rifles, and tubular magazines due to their reliable stacking and extraction without jamming in non-box-fed systems. Notable examples include the , used in U.S. Springfield rifles from 1873, and the 7.62x54R, a rimmed round employed in Soviet Mosin-Nagant rifles with an exceeding 1,000 meters. In contrast, rimless cartridges dominate modern military and civilian applications, such as the 9x19mm Parabellum in pistols and submachine guns, offering smoother feeding in box magazines and automatic firearms while enabling precise headspacing on the case shoulder or mouth. Semi-rimmed variants, like the 7.7x58mm used in Japanese Type 92 machine guns, serve as a transitional but are less common today due to potential feeding inconsistencies. A specialized subset involves rimfire cartridges, where the priming compound is distributed within the rim's annular cavity rather than a central primer, allowing ignition by a striking the rim's edge; this configuration is almost exclusively rimmed and is prevalent in low-powered small-caliber ammunition like the .22 Long Rifle. While rimmed and rimfire designs persist in hunting, target shooting, and historical replicas, the shift toward rimless and centerfire systems in post-World War II firearms reflects advancements in reliability, magazine capacity, and automation.

Fundamentals

Definition

In the context of firearms, the rim refers to the thickened, flanged edge at the base of a metallic cartridge case, formed by expanding or machining the material outward from the case body to create a protruding lip. This structural feature provides a mechanical interface for handling and positioning the cartridge within the firearm's chamber or magazine. Standards organizations like SAAMI define rim dimensions such that, for rimmed cartridge types, the rim diameter exceeds that of the case body to ensure proper engagement; for instance, in the .22 Long Rifle cartridge, the rim diameter measures 7.06 mm while the case body diameter is 5.74 mm. The rim is distinct from other cartridge base elements, including the extractor groove—a narrow circumferential channel machined into the case head ahead of the rim in rimless designs to facilitate extraction—and the area, which encompasses the stamped markings on the case base indicating , manufacturer, and year. Cartridge rims are commonly constructed from or , with prized for its , resistance, and ability to expand and contract reliably under firing pressures, thereby enhancing overall case and supporting multiple reloads; , conversely, offers economic advantages but can be less forgiving on extractors due to its relative brittleness.

Function in Firearms

The rim of a cartridge serves several critical mechanical functions in firearm operation, primarily headspacing, extraction, and ejection. Headspacing refers to the positioning of the cartridge within the chamber to ensure proper alignment and prevent excessive movement during firing; for rimmed cartridges, the forward edge of the rim contacts the chamber's recoil ledge or the rear of the barrel, establishing the correct cartridge depth and maintaining consistent bullet-to-rifling distance for accuracy and safety. In revolvers, this rim contact prevents the cartridge from over-inserting into the cylinder chambers, ensuring reliable ignition and pressure containment. Extraction involves the firearm's extractor claw engaging the rim to pull the spent cartridge case from the chamber after firing; the extractor's hook grips the rim's outer edge, allowing the bolt or slide to retract the case reliably, even under high pressure or fouling conditions. Ejection follows, where the case, still held by the extractor, contacts a fixed or spring-loaded ejector that strikes the case's side, flinging it clear of the firearm; the rim's flange facilitates this interaction by providing a stable hold point during the process. Rims also enable reliable feeding mechanisms in various designs, particularly those using magazines or cylinders. In revolver cylinders, the protruding rim allows individual cartridges to seat securely in each chamber without slipping forward, while the cylinder's star extractor simultaneously engages all rims for unified extraction during reloading. For tubular magazines in lever-action rifles, rimmed cartridges stack sequentially with each rim positioned forward of the one below, preventing slippage along the tube's length and ensuring smooth carrier lift into the chamber under spring tension; this leverages the rim as a contact point for the carrier mechanism to advance the next round reliably. In box magazines, proper stacking requires the upper cartridge's rim to align in front of the lower one's to maintain feed and avoid misalignment during or vibration. However, rimmed cartridges present challenges in multi-round loading, notably rimlock, where the rims of adjacent cartridges overlap or interlock improperly, impeding feed and causing failures to chamber. This issue arises primarily in box-fed designs when cartridges shift during handling or cycling, with the protruding rim of one catching behind another, locking the stack and halting operation; it is mitigated by careful loading orientation but limits reliability in high-speed semi-automatic actions. Rimmed cartridges are thus highly compatible with manual actions like revolvers and lever-actions, where the rim supports direct extraction and headspacing without complex feed geometry, whereas rimless designs predominate in semi-automatic firearms to ensure smoother, uninterrupted magazine feeding and reduce such jamming risks.

Rim Configurations

Rimmed

A rimmed cartridge is characterized by a base flange where the rim diameter is greater than that of the case body and neck, forming a protruding lip that enables mechanical extraction by engaging directly with the firearm's extractor without requiring an additional groove. This design provides reliable headspacing on the forward face of the rim, ensuring consistent cartridge positioning in the chamber for safe and accurate firing, particularly in revolvers and bolt-action rifles. Manufacturing rimmed cases is cost-effective due to the simpler tooling needed for the flanged base compared to more complex configurations. The advantages of rimmed cartridges include straightforward extraction in cylinder-based revolvers, where the rim allows positive engagement and ejection, and compatibility with tube-fed rifles, such as lever-actions, where cartridges stack base-to-rim without feeding interruptions. This configuration supports efficient operation in low-pressure applications and promotes economical production for high-volume calibers. However, disadvantages arise in stacked loading scenarios, like box magazines, where the protruding rims can cause "rimlock"—a malfunction in which one cartridge's rim catches on the rim of the round below, leading to feeding failures. Consequently, rimmed designs are less suitable for high-capacity, semi-automatic magazines that prioritize smooth, stacked feeding. Representative examples illustrate these traits: the .22 Long Rifle (.22 LR), a rimfire staple, features a rim diameter of approximately 0.270 inches versus a case body diameter of 0.226 inches, enabling reliable function in tube-fed rimfire rifles and pocket pistols. The revolver cartridge has a rim diameter of 0.440 inches compared to a 0.379-inch case body, supporting robust extraction in double-action revolvers while headspacing precisely on the rim. Similarly, the rifle round, with a 0.506-inch rim and 0.421-inch case body, excels in lever-action tube magazines for hunting, though it risks rimlock in modern box-fed designs.

Rimless

Rimless cartridges feature a case head where the rim is flush with or slightly smaller than the body , eliminating any protruding . Instead of relying on a rim for extraction, these cartridges incorporate an extractor groove machined into the case head just forward of the rim, allowing the firearm's extractor to engage securely for reliable removal from the chamber. This facilitates headspacing primarily at the case or , ensuring consistent positioning during firing. The primary advantages of rimless cartridges stem from their streamlined profile, which enables smooth stacking and feeding in box magazines without the risk of rimlock that can occur with protruding rims. They are particularly well-suited for semi-automatic pistols and , where reliable under rapid fire is essential, as the uniform promotes consistent alignment and reduces jamming. Additionally, the absence of a protruding rim allows for better of high-pressure loads, as the case can be designed with reinforced walls and precise headspacing to handle elevated chamber pressures without structural compromise. Despite these benefits, rimless cartridges demand precise of the extractor groove to ensure proper engagement, as inconsistencies can lead to extraction failures. Improper headspacing, often due to tolerances or in the , heightens the risk of case head separation, where excessive forward movement under pressure causes the case to split near the head, potentially releasing hot gases into . Prominent examples of rimless cartridges include the (commonly known as 9mm Luger), , and , which are widely used in handguns, rifles, and military applications. The extractor groove depth in these designs typically ranges from 0.02 to 0.05 inches, providing sufficient purchase for the extractor while maintaining case integrity.

Semi-Rimmed

Semi-rimmed cartridges feature a rim with a slightly larger than the case body, typically protruding by 0.01 to 0.03 inches, which provides a partial for extraction while maintaining a profile closer to rimless designs. This configuration includes an extractor groove beneath the rim, enabling the cartridge to function in mechanisms designed for either rimmed or rimless by offering a compromise in headspacing and feeding reliability. The primary advantages of semi-rimmed designs include enhanced compatibility with a range of firearms, such as semi-automatic pistols and submachine guns, where the modest rim aids extraction without the stacking issues common in fully rimmed cartridges. This reduces the risk of rimlock in magazines compared to more pronounced rims, facilitating smoother feeding in blowback-operated systems. Additionally, the design supports reliable headspacing on the case mouth or shoulder in many applications, making it suitable for compact handguns and military sidearms. However, semi-rimmed cartridges offer less secure headspacing than fully rimmed types, as the subtle protrusion can lead to inconsistencies in chambering under high-pressure conditions. Manufacturing these cases also introduces added complexity, requiring precise control over the rim's dimensions to avoid feeding malfunctions or identification errors with rimless ammunition. Representative examples include the , which has a rim diameter of approximately 0.374 inches compared to a case body of 0.368 inches, and the , valued for its use in submachine guns like the due to reliable extraction in rapid-fire scenarios. Other notable instances are the and , which exemplify the design's application in early 20th-century semi-automatic pistols.

Rebated

A rebated rim cartridge features a case design in which the rim diameter is smaller than the diameter of the case body at the base, creating a stepped profile that primarily facilitates extraction by the firearm's extractor claw. This configuration often includes an extractor groove cut into the case just forward of the rim to aid in reliable engagement and removal from the chamber. The design enables the use of longer cartridge cases within standard bolt-action or actions without requiring modifications to the bolt face or well, as the smaller rim matches existing extractor dimensions while allowing a wider body for increased powder capacity. One key advantage of the rebated rim is its compatibility with high-capacity magazines in rifle platforms, such as AR-15 derivatives, where the smaller rim aligns with standard .223 Remington/5.56mm bolt faces, permitting the chambering of larger-caliber rounds without altering the receiver. In bolt-action , it supports oversized bullets and higher velocities by accommodating broader case bodies in unmodified standard-length actions, enhancing performance for long-range or big-game applications. However, the smaller rim can complicate extraction, as the reduced surface area for the extractor increases the risk of slippage or failure, particularly in adverse conditions like heavy or carbon buildup, leading to potential malfunctions such as bolt override. Prominent examples include the .404 Jeffery, a classic big-game cartridge developed in 1905 with a rim diameter of approximately 0.540 inches compared to a case base of 0.544 inches, allowing it to fit standard Mauser 98 actions for dangerous game hunting in Africa. Another is the 7mm Remington Ultra Magnum, introduced in 2001, featuring a 0.532-inch rim against a 0.550-inch case base, which provides substantial powder capacity (around 100 grains) for high-velocity 7mm projectiles in long-action rifles. These designs have been adapted for modern magnum loads to achieve superior ballistic performance in conventional platforms.

Belted

A belted cartridge features a raised belt of metal encircling the case body just forward of the extractor groove and rim, providing a distinct ledge for headspacing rather than relying solely on the case shoulder or rim. This belt, typically formed by thickening the wall, protrudes approximately 0.020 to 0.050 inches beyond the case body diameter and stands about 0.222 inches high, creating a reinforced ring that supports the cartridge in the chamber during high-pressure firing. Unlike a true rim, the belt is not at the extreme base but positioned ahead of it, often in conjunction with a rebated rim that is smaller in diameter than the belt itself, allowing compatibility with magazine-fed actions. The primary advantage of the belted design lies in its secure headspacing for high-pressure magnum cartridges, where the forward belt face contacts the chamber's corresponding ledge to precisely control cartridge position and prevent excessive forward movement under pressure exceeding 60,000 psi. This configuration minimizes case blowout risks by containing expansion forces at the belt, enhancing safety in bolt-action rifles designed for dangerous game or long-range shooting. Additionally, it enables reliable extraction and compatibility with multi-round magazines by substituting for a protruding rim, reducing the likelihood of rim-lock in stacked feeding systems. However, manufacturing belted cases demands precise forming processes, as the belt must be uniformly rolled or drawn from the case wall to maintain consistent dimensions and avoid weak points that could lead to failure under repeated high-pressure cycles. Over time, especially in reloading scenarios, the belt can experience setback, where the case body stretches forward relative to the belt, potentially causing headspace inconsistencies, excessive brass flow, or separation after 5-10 firings if not monitored with tools like a case length gauge. This design may also slightly hinder smooth feeding in some bolt-action rifles due to the belt's added width, occasionally reducing magazine capacity by one round compared to beltless equivalents. Representative examples include the , introduced in 1912 as one of the first belted designs for , with a belt diameter of approximately 0.532 inches and protrusion of approximately 0.020 inches to handle pressures up to 62,000 psi. Similarly, the employs a belted case with comparable belt specifications, offering versatile performance in medium to large game applications while maintaining headspacing integrity in modern rifles.

Historical Context

Origins

The concept of rims in firearm cartridges emerged in the 19th century as part of the transition from loose powder and ball loading to self-contained ammunition, primarily through the invention of rimfire designs. French is credited with creating the first practical rimfire cartridge in 1845, a small .22-caliber round consisting of a or foil case with a priming compound distributed around the rim for ignition. This innovation marked a significant step away from earlier cartridges, which had been used since the 16th century but were prone to moisture damage and unreliable sealing. Building on Flobert's work, American inventors Horace Smith and advanced rimmed cartridge technology in 1854 by patenting an improved rimfire design tailored for revolvers, which featured a more robust metallic case to contain powder and projectile. This patent (U.S. Patent No. 11,496) enabled the production of repeating firearms and addressed the limitations of prior paper-based systems by using brass rims for better durability and extraction. By the early , the shift to fully metallic rimmed cartridges became widespread, replacing paper constructions that had dominated muzzle-loading and early breech-loading arms, as brass provided superior resistance to the corrosive effects of black powder residues. A key milestone was the introduction of the .22 Short rimfire cartridge in 1857 by for their Model 1 , recognized as the first commercially successful rimmed cartridge in the United States, propelling a 29-grain with 4 grains of black powder. The design of these early rims was influenced by the volumetric demands of black powder, which required larger case capacities and thus broader rims to ensure reliable headspacing and ignition without excessive pressure buildup in low-velocity loads. The primary purpose of these rim configurations was to facilitate rapid reloading in breech-loading firearms, exemplified by the introduced in 1860, which used the .56-56 rimfire cartridge to hold seven rounds in a tubular magazine for efficient combat use.

Evolution and Modern Use

In the early 20th century, the rise of semi-automatic firearms drove a shift toward rimless cartridge designs, which facilitated reliable feeding from box magazines without the interference common in rimmed types. John Browning's , introduced in as a truly rimless, straight-walled cartridge, was specifically engineered for the Colt to ensure smooth operation in self-loading mechanisms, marking a pivotal advancement over earlier semi-rimmed pistol rounds like the . This transition gained momentum during and II, as military adoption of semi-automatics favored rimless configurations for their compatibility with high-capacity magazines and reduced risk of malfunctions. Concurrently, belted designs emerged for high-powered rifle cartridges; introduced the belted in 1912 to provide precise headspacing in bolt-action rifles using propellant, where shallow shoulders limited reliable extraction, influencing subsequent magnum developments like the .300 H&H in 1920. Standardization efforts further shaped rim evolution, with the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), formed in 1926, establishing voluntary compliance standards for cartridge and chamber dimensions, including rim specifications, to promote safety and interchangeability across manufacturers. The European counterpart, the Commission Internationale Permanente (CIP), adopted similar protocols in the mid-20th century, harmonizing global designs and reducing variability in rim profiles for both civilian and applications. Modern innovations have built on these foundations, incorporating materials in to significantly reduce weight compared to cases, enhancing portability for exercises without compromising function in semi-automatics. Hybrid brass-polymer cases, such as those in SIG Sauer's 6.8x51mm Fury developed for the U.S. Army's program, combine metallic rims for extraction reliability with lightweight polymer bodies, supporting modular rifle platforms that adapt across calibers. Today, rimmed cartridges remain prevalent in rifles with tubular magazines, such as lever-actions chambered for the , where the protruding rim aids stacking and extraction in traditional designs favored by big-game . In contrast, rimless cartridges dominate military applications, exemplified by the 5.56x45mm , a bottlenecked round optimized for assault rifles like the due to its seamless magazine feeding and compatibility with selective-fire mechanisms. Rebated-rim designs, featuring a rim diameter smaller than the case body, find niche use in precision long-range shooting; the (RUM), introduced in 2001, leverages this configuration to maximize powder capacity in standard bolt faces while delivering flat trajectories beyond 1,000 yards for competitive and varmint applications. Ongoing trends reflect a decline in semi-rimmed cartridges, which once bridged rimmed and rimless needs but now face obsolescence due to feeding inconsistencies like nosedive in modern semi-automatics, leading manufacturers to favor full rimless profiles for versatility. Environmental regulations, particularly California's 2014 ban on lead-based hunting ammunition—expanded statewide by 2019—have spurred lead-free alternatives, primarily in bullets and primers, to minimize ecological contamination and comply with stricter wildlife protection laws. These shifts underscore a broader push toward sustainable, high-performance designs amid stricter wildlife protection laws.

References

  1. https://apps.dtic.mil/sti/trecms/pdf/AD1008964.pdf
  2. https://nij.ojp.gov/nij-hosted-online-training-courses/firearms-examiner-training/glossary
  3. https://saami.org/glossary/rimfire-cartridge/
  4. https://www.theballisticassistant.com/saami-rimfire-cartridge-catalog/
  5. https://www.gunsandammo.com/editorial/case-metallurgy/504155
  6. https://www.bighornarmory.com/wp-content/uploads/2020/04/Notes-on-the-Care-and-Feeding-of-Lever-Guns-1.pdf
  7. https://www.americanrifleman.org/content/marching-to-their-own-beat-smith-wesson-m-p-tempo-pistols/
  8. https://patentimages.storage.googleapis.com/f5/bf/b7/f21b701ffb570f/US20140096671A1.pdf
  9. https://www.americanrifleman.org/content/brush-busters-are-back/
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  52. https://www.themeateater.com/hunt/firearm-hunting/10-best-modern-hunting-cartridges
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  54. https://gundigest.com/more/how-to/300-remington-ultra-magnum
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  56. https://www.nathab.com/blog/california-is-the-first-state-to-ban-lead-ammunition-should-all-states-follow-suit
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