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 caliber conversion sleeve , also known as a cartridge conversion sleeve and as a supplemental chamber , is a device commonly sold for several 0.30-caliber-chambered rifles.[1] These devices enable the bolt-action rifles for which they are made to use 32 ACP cartridges in addition to the cartridges in the caliber for which the rifles are chambered. The caliber conversion sleeves resemble fired cartridge cases in the rifle calibers for which they are made, but are made of steel instead of brass, and have an opening where the primer would normally sit into which the 32 ACP cartridge is inserted, prior to chambering the combined 32 ACP round and caliber conversion sleeve in the rifle. No modifications to the rifle are required.

Three such 0.30-caliber rifles commonly have such caliber conversion sleeves available, for rifles chambered in the following cartridges:

.308 Winchester, which use bullets that are 0.308 inches in diameter
30-06, which use bullets that are 0.308 inches in diameter
.303 British, which use bullets that are 0.311 inches in diameter.

Caliber conversion sleeves work because the common 32 ACP bullet is 0.3125 inches in diameter, and will fire in practice in rifles with only slightly smaller bores than the 32 ACP bullet diameter, provided the difference in the rifle chamber size is properly compensated for with a caliber conversion sleeve.

Military .303 British 174g FMJ cartridge

There are limitations on the use of such caliber conversion sleeves. For example, the .303 British 174gr round normally exits the rifle muzzle at a velocity of approximately 2440 ft/s. When firing the 71 gr 32 ACP in the caliber conversion sleeve in such a rifle, the 32 ACP round will exit the muzzle at a velocity of only around 1100 ft/s. This reduces the effective range considerably, often to only 50 yards or so, and provides much less foot-lbs on target. The sights are additionally only approximately accurate, as the bullet mass (in grains) and the nominal rifle cartridge velocity have all been assumed in the calibration of the sights on the rifle. The caliber conversion sleeve is also typically capable of being loaded only one round at a time, effectively converting a repeater rifle into a single shot rifle.

Users also need to be aware that a squib may often result, as cheap 32 ACP FMJ cartridges may not have enough energy for the 32 ACP bullet to clear the barrel. This is especially dangerous if a high-powered rifle round is then loaded and fired after a 32 ACP bullet has lodged in the barrel.

However, the benefits when proper 32 ACP ammunition is used are that the recoil and noise of the rifle is greatly reduced, the price per round fired is much cheaper ($0.20/rd, or less, vs. $0.50-$1.00/rd), and, in a survival situation in which only a limited number of rifle rounds are available for hunting, the 32 ACP is certainly adequate for small-game hunting out to 50 yards. Hence, the caliber conversion sleeve can provide additional survival hunting rounds if one additionally carries a box or two of lighter 32 ACP cartridges in addition to carrying the usual rifle cartridges. In addition, it often becomes possible to use an indoor pistol range when firing 32 ACP cartridges in a hunting rifle chambered for a much more powerful 0.30-caliber rifle, opening up more firing range opportunities for practicing with powerful rifles in dense urban areas.

Caliber conversion sleeves for firing 32 ACP cartridges in 30-caliber rifles sell for approximately $16-$20 (2006) from major US gun parts distributors.

Occasionally, chromed or blued supplemental chambers for firing .32 S&W (0.315-inch diameter bullets) in 30-caliber rifles are also seen; these devices often date to the late 19th Century, and were made by Winchester, among others.[2] Similarly, there were also comparable devices sold for the 6.5 x 54 Mannlicher Schoennauer Model 1903 to reduce the round down to .25 auto that were widely available until roughly WW II. [3] The comparable devices for these older cartridge conversions are no longer commonly sold by major US gun parts distributors.