The introduction of the .22 Magnum (Winchester Magnum Rimfire—WMR) occurred in 1959. It was developed as a rimfire cartridge that would possess a velocity close to that of a centerfire. It is loaded with either jacketed hollow-point or full metal-jacketed bullets. Both handguns and rifles are chambered for this cartridge. The .22 Magnum has a larger cartridge case diameter than the other rimfire cartridges and will not chamber in weapons chambered for the standard .22 rimfire cartridges. The .22 Short, Long, and Long Rifle cartridges will fit loosely in a weapon chambered for the Magnum cartridge. They ordinarily will not fire; if they do, the cases will split.

The .22 Magnum cartridge is loaded with an 0.224-in. bullet compared with the 0.223-in. bullet used in the other .22 rimfire cartridges. Some rimfire revolvers have interchangeable cylinders designed so that one cylinder is for the ordinary .22 rimfire cartridges and the other is for the .22 Magnum. The barrel has a groove diameter of 0.224-in., i.e., that of the Magnum bullet. When non-magnum rimfire ammunition is fired, the 0.223-in lead bullet expands, as a result of gas pressure, to fill the rifling.

.22 Magnum cartridges are loaded with either a 40 gr. full metal-jacketed bullet or a jacketed hollow-point (JHP) bullet whose weight varies depending on the manufacturer. Winchester loads a 40 gr. JHP; Federal, a 50 gr. and a 30 gr.; CCI, a 40 gr. and a 30 gr. The CCI 30 gr. JHP bullet has a pentagonal hole in its tip and is loaded in a nickel-plated case. Muzzle velocity from a rifle is 2200 ft/s; 1910 ft/s; and 1650 ft/s for the 30 gr., 40 gr. and 50 gr. bullets, respectively, with velocities of 500 to 600 ft/s less for handguns. Muzzle energy in the rifles is between 300 and 325 ft-lbs.

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