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.
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. The Hammond Game Getter works around this problem in its sleeves for .22 rimfire by creating an eccentric chamber that places the rim of the .22 cartridge in the center of the original chamber, thus allowing the firing pin to strike it directly.
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.
Read more about this topic: Caliber Conversion Sleeve