Caliber conversion sleeve

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A 32 ACP FMJ cartridge, a 32 ACP FMJ cartridge in a blued .303 British supplemental chamber, and a .303 British 180g FMJ cartridge (left to right)

A number of different devices exist whose function is to non-permanently alter a firearm to 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.

1 Altering cartridge length
2 Altering the cartridge diameter
3 Altering the caliber
- 3.1 Chamber length sleeves
- 3.2 Greater than chamber length sleeves
4 Shotguns
5 Uses
6 References

[edit] 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 Springfield to 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^[1].

[edit] Altering the cartridge diameter

The next level in complexity involves altering the diameter of the cartridge used, typically allowing a short, straight walled handgun cartridge 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 caliber of the cartridges must match. The most commonly encountered supplemental chambers are for .30 caliber (7.62 mm) rifle cartridges, and use .32 ACP or other similarly sized cartridges. While this seems like it wouldn't work (.30 vs. .32 caliber), the .32 caliber handgun cartridges actually chamber bullets the same diameter as .30 caliber rifles (see heeled bullet for why this is so).

Supplemental chambers in .22 caliber, and potentially .17 caliber as well, pose a special problem not shared by larger calibers. Rimfire cartridges, 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 centerfire design. 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 Arms Contender, 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.

[edit] 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 rifling to stabilize the bullet; the other types rely on the rifling in the firearm's barrel.

[edit] 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).

[edit] Greater than chamber length sleeves

Sleeves that exceed the chamber length are generally used in break open actions, 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^[2] 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^[3].

[edit] Shotguns

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.

Since 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^[4]. Many makers offer longer inserts, though due to the variations in shotgun bore diameters, these usually require custom manufacture^[5].

[edit] Uses

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^[6].

Some European conversions exist for special gallery cartridges, similar in concept to the .22 CB rounds, which use a primer but no gunpowder. These adapters were chambered for centerfire versions of these tiny rounds, such as the 4mm 4mm Ubungsmunition cartridge^[7]. Gallery cartridges such as these were intended for use in indoor target practice, and are similar in power and report to an airgun.