The .22 Hornet is a low-end varmint and predator centerfire rifle cartridge. It is considerably more powerful than the .22 WMR and the .17 HMR, achieving higher velocity with a bullet twice the weight. The Hornet also differs very significantly from these in that it is not a rimfire round but a centrefire one. This makes it handloadable and reloadable, and thus much more versatile.
It is the smallest commercially available .22 caliber centerfire round, although Remington makes .17 caliber centerfire rounds.
The .22 Hornet fills the gap between such popular varmint/predator cartridges as the .22 WMR and the .223 Remington. In regards to muzzle velocity, muzzle energy and noise, it is well suited to varmint and predator control in relatively built-up areas. The Hornet's virtual absence of recoil has made it even quite popular among deer hunters in some areas, although it is generally regarded as very underpowered for deer unless bullet placement is absolutely precise. Many jurisdictions such as the UK and most states in the USA currently prohibit the Hornet for use on deer.
The .22 Hornet's ancestry is generally attributed to experiments done in the 1920s using the black-powder .22 WCF at Springfield Armory.  Winchester adopted what had so far been a wildcat cartridge in 1930, producing ammo for a cartridge for which no commercially-made guns yet had been built. It wasn't until 1932 that any company began selling commercially-made guns for the cartridge.
Older guns generally have a slower twist rate of 1-16" (or one turn in every 16 inches of barrel length) for lighter bullets with a .223 caliber dimension. Newer guns feature a faster 1-14" twist for 40 to 45 grain bullets in the more standard .224 caliber.
Beginning during World War II, aircrew survival rifles in .22 Hornet were developed and issued by the U.S. military. They typically were bolt-action rifles with telescoping stocks or break-open rifle/shotgun over-under designs. 
Rifles are currently (2007) being chambered in .22 Hornet by Ruger, New England Firearms, CZ and various other mass-market manufacturers. Most current-production rifles in .22 Hornet are either bolt-action or single-shot designs, with the exception of a very few "survival" rifle/shotgun over-under designs such as the Savage Model 24 from Savage and a few European-made kipplaufbreak-action, single-shot rifles. It is possible to get an extremely-accurate new .22 Hornet rifle for as little as US$200, although prices can go very much higher for rifles made by custom riflemakers and the specialist London and European trade.
Revolvers have been produced in .22 Hornet by Taurus, Magnum Research, and others. Single-shot pistols in .22 Hornet have been made by Thompson. (Power levels are somewhat less for this cartridge in short-barrelled handguns than in rifles.)
In Great Britain, the .22 Hornet was extremely popular among specialist roe deer stalkers in the early 20th century; but the calibre was outlawed by the 1963 Deer Act owing to inadequately low bullet energy, and has fallen considerably in British popularity since then. Wildcat variants of the .22 Hornet, such as the .22 K-Hornet, can boost bullet velocity and energy considerably above factory .22 Hornet levels, but performance still falls short of what is deer-legal in any part of the United Kingdom.
Factory ammunition is widely available from all major manufacturers, generally with bullets weighing 34, 35, 45, or 46 grains (2.2, 2.3, 2.9, 3.0 g), with bullets invariably either hollow point or soft point. Muzzle velocity typically is in the 2,500 to 3,100 ft/s (760 to 940 m/s) range, and muzzle energy is just over 700 ft·lbf (950 J) for factory ammo fired from a rifle. (Velocities and energies are less when Hornet ammunition is fired from short-barrelled firearms.)
Published handload data from major handloading-product companies shows how versatile the .22 Hornet can be. For instance, it is easy to use these data to load .22 Hornet ammo with heavier bullets than the major manufacturers offer, to produce loads that are significantly more powerful than the .22 WMR but that are no noisier than most commercially loaded .22 Long Rifle high-velocity ammo. According to the Hodgdon reloading data, the heavier bullets show a serious affinity for Lil'Gun powder to produce much higher velocities than other powder with heavy bullets in this small case.