220 Swift also called 5.56×56mmSR is a semi-rimmed rifle cartridge developed by Winchester and introduced in 1935 for small game and varmint hunting. It was the first factory-loaded rifle cartridge with a muzzle velocity of over 1,200 m/s (4,000 ft/s), just under Mach 4.
The velocity of the cartridge ranges from 2000 km/h (1200 mph or 550 m/s) up to about 4500 km/h (2800 mph or 1250 m/s). 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. When introduced it was 1,400 ft/s (430 m/s) faster than its nearest varmint-hunting competitor, which was the .22 Hornet (also .224 caliber).It was found to be an extremely accurate cartridge as well. The .220 Swift remains the fastest commercial cartridge in the world, with a published velocity of 1,422 m/s (4,665 ft/s) using a 1.9 grams (29 gr) bullet and 2.7 grams (42 gr) of 3031 powder.
Due to its very high velocity its bullet drop allows precise 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 1,200 metres per second (4,100 ft/s). Handloaders could marginally improve on this but only at maximum loads. The Swift can be loaded with light bullets to reach 1,300 m/s (4,400 ft/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.
HISTORY ABOUT 220 SWIFT
.220 Swift was developed in 1934–35 by Grosvenor Wotkyns who necked down the .250-3000 Savage as a means of achieving very high velocities. However the final commercial version developed by Winchester is based on the 6mm Lee Navy cartridge necked down, but besides inheriting headspacing on its rim from the parent, a feature already considered obsolete by 1930s, the protruding rim which complicates loading was even made larger to fit with 12mm-wide .30-06 bolt faces. The .220 Swift was developed by Winchester and introduced in 1935 as a new caliber for their Model 54 bolt-action rifle. When the Winchester Model 70 bolt action was first issued in 1936, the .220 Swift was one of the standard calibers offered and continued to be until 1964 when it was discontinued.
The Swift has the dubious privilege of being possibly the most controversial of all the many .224 in caliber 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 precision after well in excess of 4,000 shots.More popular, however, is the smaller and slightly lower velocity .22–250.