Rim (firearms)

Source: Wikipedia, the free encyclopedia.
Not to be confused with Rimfire ammunition.

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

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

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, which 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

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

Desert Eagle in .357 Magnum or .44 Magnum. And, of course, 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

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.

primer, as is commonly used in centerfire cartridges.[1]

Under the

M1917 service revolvers
.

Examples of rimmed handgun cartridges include the

.45-70 Government.[2]

Rimless 9mm Parabellum pistol cartridges

Rimless

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

Colt or Smith & Wesson M1917 revolvers in .45 ACP).[3]

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

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

6.5x52mm Carcano, 6.5×54mm Mannlicher–Schönauer, 6.5×55mm Swedish, .308 Winchester, .30-06 Springfield, 7.65x53mm and 7.92×57mm Mauser.[3]

Semi-rimmed

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.[3]

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.[4]

Examples of semi-rimmed handgun cartridges are

6.5×50mm Arisaka, .308 Marlin Express, .338 Marlin Express, and .444 Marlin.[3]

Rebated

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.[3] 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

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.[5][6]

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)

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.[7][6]

The only known Shotgun shell using rebated rims is the 12 Gauge RAS12, specially made for the RAS-12 semi automatic shotgun.[8]

The

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.[7]

Other rebated rifle cartridges include the

.375 SWISS P
.

Rebated cartridges are used for a different reason on

"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 have been used in the T168 autocannon prototype, the 30x120mmRB 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

Main article:
Belted case
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

.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.[9]

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.[10]

See also

References

  1. ISBN 978-1-4402-4659-3. {{cite book}}: |work= ignored (help
    )
  2. ISBN 978-1-61608-222-2
    .
  3. ^
    ISBN 978-1-4665-8881-3
    .
  4. ISBN 0-87349-281-1
    .
  5. ISBN 978-1-63450-966-4
    .
  6. ^
    ISBN 978-1-4402-4642-5
    .
  7. ^
    ISBN 978-1-4402-3998-4
    .
  8. ^ "Shotshell type ammunition usable in magazine-fed firearms, and methods of manufacturing such shotshell type ammunition".
  9. ISBN 978-1-4402-3848-2
    .
  10. ISBN 978-1-4402-4658-6
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Rim_(firearms)&oldid=1219645152"