- Caliber conversion sleeve
A number of different devices exist whose function is to non-permanently alter a
firearmto allow it to fire a different cartridge than the one it was originally designed to fire. These devices are called by many names, including caliber conversion sleeve, cartridge conversion sleeve, supplemental chamber, chamber insert, and subgauge inserts or subgauge tubes. This article will break the various conversions down into types by the degree to which they alter the cartridge used by the firearm.
Altering cartridge length
The simplest conversion is one that alters the length of the cartridge used, allowing a shorter but otherwise similarly dimensioned cartridge to be fired. Commonly called "chamber inserts", these conversions resemble the front of the case, and are inserted into the firearm by placing over the new cartridge and inserting both into the chamber of the firearm. This seats the chamber insert into the front of the chamber, where it remains after firing. Once inserted, the chamber insert will remain in place until removed with the use of a
stuck case remover. The most commonly encountered chamber inserts are ones designed to convert .30-06 Springfieldto the shorter 7.62 x 51 mm NATO. Since the chamber insert remains in the chamber, this type of conversion will function in semi-automatic firearms, and is commonly used in military surplus arms such as the M1 Garand, allowing the use of often less expensive surplus military ammunition [ [http://www.shootingtimes.com/longgun_reviews/st_0302_springfield/ Shooting Times: Springfield M1 Garand .308 ] ] .
Altering the cartridge diameter
The next level in complexity involves altering the diameter of the cartridge used, typically allowing a short, straight walled
handguncartridge to be used in a rifle. These are often called "supplemental chambers", and the entire supplemental chamber is treated like a cartridge, being loaded and ejected as a unit with the smaller cartridge. Since the barrel of the firearm is used, the caliberof the cartridges must match. The most commonly encountered supplemental chambers are for .30 caliber (7.62 mm) rifle cartridges, and use .32 ACPor other similarly sized cartridges. While this seems like it wouldn't work (.30 vs. .32 caliber), the so-called ".32 caliber" bullet is the same diameter as a ".30 caliber" rifle bullet.
Supplemental chambers in .22 caliber, and potentially .17 caliber as well, pose a special problem not shared by larger calibers.
Rimfirecartridges, with their low cost, noise, and recoil, are ideal for use in a supplemental chamber, except for the fact that the firearm in question is almost certainly a centerfiredesign. This means that supplemental chambers that use a rimfire cartridge must also provide a special offset firing pin. This is a metal insert that fits behind the rimfire cartridge, and has an appropriate projection to act as a firing pin. When this insert is hit by the firearm's firing pin, it is pushed forwards, which causes it to crush the rim of the rimfire cartridge, igniting it. Use of the centerfire to rimfire conversion requires a longer case, and so is not suitable for short cartridges such as .22 Hornet. A notable exception to this is the Thompson Center ArmsContender, which has both centerfire and rimfire firing pins which can be selected with the turn of a switch, and does not require a centerfire to rimfire converter.
Since the supplemental chambers are approximately the size of a normal loaded cartridge, they will usually feed from a magazine, though they will not provide sufficient energy to cycle an automatic action.
Altering the caliber
The most complete transformation is offered by the "caliber conversion sleeve". These include not only a new chamber, but a new barrel as well, allowing a smaller diameter bullet to be fired. These sleeves may be significantly longer than a loaded cartridge, if the length of the parent cartridge is not sufficient to provide the desired performance. Unlike the other types of cartridge conversions, this type incorporates
riflingto stabilize the bullet; the other types rely on the rifling in the firearm's barrel.
Chamber length sleeves
Chamber length sleeves are restricted cases where a short cartridge is used with a long parent cartridge. The sleeve is rifled up to the end of the sleeve. Since most cartridge cases are only a couple of inches long (about 5 cm) this provides limited power. On the other hand, these will function from magazines in the same way as supplemental chambers, and allow the use of the least expensive rimfire ammunition in firearms chambered in calibers larger than .22 (5.56 mm).
Greater than chamber length sleeves
Sleeves that exceed the chamber length are generally used in
break openactions, which allow easy insertion and removal. Like supplemental chambers, caliber conversion sleeves completely surround the new cartridge case, but cannot be ejected or fed from a magazine, so they only offer a single shot per barrel without manual extraction and reloading.
The calibers supported by caliber conversion sleeves are limited by the difference between the calibers. The sleeve's barrel must be thick enough to provide structural integrity to the barrel, and so requires a large enough internal barrel diameter to hold the new barrel. One manufacturer has a .40 caliber (10 mm) minimum diameter for these inserts [ [http://www.mcace.com/rifleinserts.html Insert Barrels For Rifles And Single Shot Pistols! ] ] in .22 rimfire caliber.
Some manufacturers offer caliber conversion sleeves for
shotguns, which convert a shotgun into a rifle with the use of a rifled barrel [ [http://www.mcace.com/shotguninserts.htm Our Complete Line Of Shotgun Inserts To Increase The Versatility Of Your Shotgun ] ] .
Inserts for shotguns are called "subgauge inserts", and function in much the same manner as inserts for handguns or rifles. However, due to the nature of shotguns, the implications of using an insert differ.
shotgun shells are all straight walled, a change in chambering means a change in diameter of the shell. However, since subgauge inserts are designed to be used with shot, not slugs, the shotgun's barrel can continue to be used, with little or no impact on patterning [ [http://www.chambermates.com/patterns.htm Chamber Mates Sub Gauge Shotgun Chamber inserts Patterns & Velocity ] ] . Many makers offer longer inserts, though due to the variations in shotgun bore diameters, these usually require custom manufacture [ [http://www.briley.com/index.asp?PageAction=VIEWPROD&ProdID=108 Companion Drop-In .410 Bore Tube Set, UltraLite - COMP410ULT ] ] .
Rifled inserts are also available, to allow the use of handgun cartridges or relatively low-power rifle cartridges.
Automatic firearm conversions
automatic firearms are often more complex than those for single shot or manual repeaters, as the difference in power available to the operating mechanism can be significant. There are a number of makers of cartridge conversions that convert various centerfire firearms to .22 Long Rifle, operating both as cartridge conversions and caliber conversions.
These conversions typically convert the firearm from
gas operationor recoil operationto a simple blowback operation. This typically involves replacing the firearm's bolt or slide with a lightweight part lacking the locking mechanism, and a reduced power spring. The magazine must also be replaced, and in the case of caliber conversions, often the barrel as well. Conversions for short recoil pistols, such as the M1911, Glock, and Beretta 92consist of a new upper assembly and magazine. Conversions for .22 caliber centerfire rifles and carbines such as the AR-15and Mini-14consist of a magazine and an insert that replaces the bolt and includes a cartridge conversion insert that goes into the chamber. Conversions for rifles and carbines of larger caliber, such as the AK-47or Thompson submachine guninclude a rifled insert barrel extending beyond the length of the chamber. [cite web |url=http://www.22lrconversions.com/ |title=Jonathan Arthur Ciener, Inc. Gun Conversions] [cite web |url=http://www.marvelprod.com/ |title=Marvel Precision]
One exception to the use of a special lightweight bolt was the Colt Service Ace, offered as a complete pistol or as a conversion kit for the M1911. While still nominally a blowback conversion, the Service Ace used a special "floating" chamber in the barrel that acted like a gas operated firearm's piston, helping push the bolt back with more force than a typical .22 Long Rifle blowback action. This additional force allowed the small .22 Long Rifle cartridge to cycle a standard weight slide, which made the pistol handle more like the standard
.45 ACPversion. [cite web |url=http://www.sightm1911.com/lib/history/colt_ace.htm |title=COLT ACE, SERVICE ACE & CONVERSIONS |author=Rick Breneman]
One interesting cartridge conversion was the
Pedersen device, which was designed to convert the bolt action Springfield 1903Mark I into a 40 shot blowback submachine gunchambering a lengthened version of the .32 ACPcartridge. The 1903 Mark I differed from the standard rifle in that had a slot cut in one side of the receiver, which to served as an ejection port for the Pedersen device. The Pedersen device replaced the bolt of the 1903, and loaded from a magazine inserted from the top right of the receiver. Intended as a "secret weapon", the device was not ready for issue before the end of World War I, and it was eventually declared obsolete without being issued, and all but a handfull of the devices were destroyed by the military. [cite web |url=http://www.odcmp.com/Services/Rifles/pipsqueak_pistol.htm |title=The Pipsqueak Pistol That Never Was... |author=Dick Culver |publisher=Civilian Marksmanship Program]
In addition to often being less expensive than the larger cartridge, the smaller cartridges offer much reduced recoil and muzzle blast. They are useful for short range target practice or killing small pests. With shotguns, the smaller shot load carried by a smaller shell increases the challenge of hitting targets, and therefore can be used as a handicap for a skilled shooter, or to increase difficulty when training [ [http://www.shotgunreport.com/TechTech/TechnicalTracts/SubgaugeLds.html Shooting Subgauge Loads ] ] .
Some European conversions exist for special "gallery cartridges", similar in concept to the
.22 CBrounds, which use a primer but no gunpowder. These adapters were chambered for centerfire versions of these tiny rounds, such as the 4mm "4mm Übungsmunition" cartridge [ [http://www.oldammo.com/november03.htm The Cartridge Collector ] ] . Gallery cartridges such as these were intended for use in indoor target practice, and are similar in power and report to an airgun.
* [http://www.mcace.com/adapters.htm MC Ace caliber adapters]
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