The .22 Winchester Magnum Rimfire, more commonly called .22 WMR, .22 Magnum, or simply .22 Mag, is a rimfire rifle cartridge. Originally loaded with a bullet weight of 40 grains (2.6 g) delivering velocities in the 2,000 feet per second (610 m/s) range from a rifle barrel, .22 WMR has also been loaded with bullet weights of 50 grains (3.2 g) at 1,530 feet per second (470 m/s) and 30 grains (1.9 g) at 2,200 feet per second (670 m/s).

The .22 WMR was introduced in 1959 by Winchester, but was not used by Winchester until the venerable Winchester Model 61 slide rifle could be chambered for it, well into 1960. By that time, Smith and Wesson and Ruger had revolvers for it, and Savage had come out with the Model 24, a .22/.410 combination rifle. It was the only successful rimfire cartridge introduced in the 20th Century.

The .22 WMR uses a larger case than the more popular .22 Long Rifle, both in diameter and length. The .22 WMR case is a lengthened version of the older .22 WRF (.22 WRF may be fired in most guns chambered for .22 WMR but not vice versa). The .22 WMR's case is thicker than that of the .22 LR, allowing higher pressures. The combination of more powder and higher pressures gives velocities over 2,000 feet per second (610 m/s) from a rifle using a 30-grain (1.9 g) bullet, and 1,500 feet (460 m) per second (460 m/s) from a handgun. A .22 WMR round will not fit into the chamber of a .22 LR firearm but is possible to chamber and fire .22 LR rounds in a .22 WMR firearm. This is potentially hazardous due to the difference in case diameter. When fired, the .22 LR case expands in the chamber, which can cause the case to split open or become stuck. Also, the long rifle bullet will travel in the magnum chamber unsupported for 0.4" before entering the barrel causing leading in the chamber.

Year 22 (XXII) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Agrippa and Galba (or, less frequently, year 775 Ab urbe condita). The denomination 22 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.