The .220 Swift is a riflecartridge developed by Winchester Repeating Arms Company and introduced in 1935. It was the first factory loaded rifle cartridge with a muzzle velocity of over 4,000 ft/s (1,200 m/s) and is still one of the fastest and most accurate factory loaded small arms cartridges, though both the .17 Remington and .204 Ruger come close with their lighter and smaller diameter bullets.
The .220 Swift uses .224 in diameter bullets, as do most of the .22 caliber centerfire cartridges.
The original prototype was based on the .250-3000 Savage case, but final designs used the 6mm Lee Navy case instead. 
The Swift has the dubious privilege of being possibly the most controversial of all the many .224 in calibre cartridges, and has inspired equal heights of praise and criticism. Traditionalists have roundly condemned it as an overbore "barrel burner" which can wear out a chromoly barrel in as few as 200-300 rounds, especially if long strings of shots are fired from an increasingly hot barrel. Its supporters have maintained that the fault lies with poor-quality barrel steels and the failure of users to remove copper fouling after firing, and point to instances of rifles with fine-quality stainless steel barrels chambered for the Swift, which have maintained sub-MOA accuracy after well in excess of 2,000 shots.
Performance is currently matched by the newer .223 WSSM but the Swift remains more popular. Even more popular however is the smaller, and slightly lower velocity .22-250.
The Swift's high-velocity performance undoubtedly comes at a price, because the high velocities and high internal firing temperatures do accelerate chamber and bore wear.  But modern metallurgy and cryogenics have vastly improved barrel life with the .220 Swift and other 4,000 ft/s ( 1200 m/s) cartridges, although they still tend to require rechambering or rebarreling much sooner than lower-velocity cartridges such as the .222 Remington and the .223 Remington.
The .220 Swift is a Delta L problem cartridge, meaning it can present unexpected chambering and/or feeding problems. The Delta L problem article explains this problem in more detail.
The Swift remains a controversial deer calibre, and its use is prohibited in many US states, and also in England, Wales and Northern Ireland for large deer such as Red, Sika and Fallow. In the cartridge's early days during the 1930s, expert Red deer stalkers such as W.D.M. Bell, the recently retired African elephant hunter, used the .220 Swift on large stags with great success, and extolled the calibre's seemingly magical killing powers, which they attributed to massive hydrostatic shock waves set up in the animal's body by the impact of the very high-velocity bullet. 
Critics of the Swift have maintained that the light 50- or 55-grain (3.24 - 3.56g) bullet leaves inadequate margin for error in bullet placement for the average deer shooter's skills, and thus invites wounding which would have otherwise have been avoidable. There is, however, little debate about the Swift's proven effectiveness on small deer species, such as Roe, provided very fast-fragmenting "varmint"-type bullets are not used.
Most factory Swift rifles come with a fairly slow twist-rate such as 1-12" or 1-14", designed to stabilize the lighter bullets popular in varmint hunting. Custom Swifts can have faster twist-rates such as 1-9" allowing them to stabilize heavy bullets, including those with a construction suitable for larger game.
P.O. Ackley maintained that the .220 Swift was a fine round for medium-large game and used it extensively for example when culling wild burros in the American West.