Caliber Conversion Sleeve - Automatic Firearm Conversions

Automatic Firearm Conversions

Conversions for 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 operation or recoil operation to 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 92 consist of a new upper assembly and magazine. Conversions for .22 caliber centerfire rifles and carbines such as the AR-15 and Mini-14 consist 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-47 or Thompson submachine gun include a rifled insert barrel extending beyond the length of the chamber.

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 ACP version.

One interesting cartridge conversion was the Pedersen device, which was designed to convert the bolt action Springfield 1903 Mark I into a 40 shot blowback submachine gun chambering a lengthened version of the .32 ACP cartridge. 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 handful of the devices were destroyed by the military.