The .220 Swift (5.56×56mmSR) is a semi-rimmed rifle cartridge developed by Winchester 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). Until the introduction of the .223 WSSM the .220 Swift was the fastest commercial cartridge in the world.
The Swift is a large cased .224 caliber cartridge and bullet that was created for small game such as prairie dogs, groundhogs and other vermin (or “varmints” in the US) such as marmots. Upon its introduction it astounded the varmint hunting world by being fully 1,400 ft/s (430 m/s) faster than its nearest competitor, which was the .22 Hornet (also .224 caliber). It was found to be an extremely accurate cartridge as well.
Due to its very high velocity its bullet drop allows dead on sighting on game such as groundhogs to ranges out to 375 yd (343 m), and it is still considered an excellent cartridge for taking varmints by experienced Swift shooters.
The original factory load from Winchester provided a 48-grain (3.1 g) bullet launched at 4,100 feet per second (1,250 m/s). Handloaders could marginally improve on this but only at maximum loads. The Swift can be loaded with light bullets to reach 4,400 ft/s (1,300 m/s). In recent times 75-grain (4.9 g) .224″ bullets have been developed for use in high velocity .22 caliber rifles for taking larger game and long-distance shooting. Heavier bullets perform best in rifles that have an appropriate rifling twist rate taking into consideration the diameter, length, and other physical properties of the projectile.