The story of this cartridge begins in around 1950, when US Army initiated the study in small arms effectiveness, that culminated in famous ORO reports by Hall (An Effectiveness Study of the Infantry Rifle) and Hitchman (Operational Requirements for an Infantry Hand Weapon). Among other conclusions, these reports called for weapon with effective range of about 300 yards, with controlled pattern dispersion of hits in bursts, to improve hit probability. One of proposed solutions was to create burst-firing small-caliber weapon firing conventional projectile. In 1952, an M2 carbine re-barreled to experimental .22 caliber round (based on shortened .222 Rem case) was studied and found promising. Various other rounds with .22 caliber projectiles, based on .30-06 and T67 (.30 / 7.62 NATO) were tseted as well. In 1957, the Armalite Ar-15 rifle, firing the newly developed .222 remington Special cartridge was tested, and in 1959 the .222 Special cartridge, developed to fulfill new military requrements for 500 meters effective fire, was renamed to .223 Remington (as a commercial offering). In 1961 US Air Force and ARPA requested a number of .223 caliber Ar-15 rifles, and military service of the new round commences in early 1962, when US AF standartised Ar-15 rifle as M16. US Army adopted the .223 cartridge as 5.56x45mm in 1963, and Remington commercially introduced the .223 as a hunting round in 1964.
Following the problems with 7.62mm M14 rifle and protracted delays with SPIW program, the M16 rifle and its .223 caliber (5.56×45) ammunition was adopted as a next US Military infantry rifle system. After much field use in Vietnam and elsewhere, and extensive trials, an updated version of the .223 cartridge, loaded with heavier SS109 bullet of Belgian design, was officially adopted in 1979 as a new NATO round, known as 5.56×45 NATO. Recent issues with insufficient stopping power of M855 5.56mm NATO ammunition in Iraq and Afghanistan (especially when firing from short-barreled carbines) resulted in development of an even heavier loading, the Mk.262, with bullet of enhanced ballistic and striking performance. Many other types of projectiles also available for this caliber in ‘military’ versions, ranging from blank and short-range tpractice and up to armour-piercing and tracer. The variety of civilian versions (intended for small-game hunting, plinking, target shooting and self-defense) is almost infinite, and this round is produced in many countries. Great many weapons, both military and civilian, are available for this caliber.
Designation Bullet weight, g Muzzle velocity, m/s Muzzle energy, J Comments
5.56×45 M193 3.5 991 1750 from standard assault rifle barrel (20″)
5.56×45 M885 4.15 930 1790 from standard assault rifle barrel (20″)
5.56×45 M885 4.15 880 1600 from carbine barrel (14″)
5.56×45 Mk.262 5 895 2000 from standard assault rifle barrel (20″)
5.56×45 Mk.262 5 772 1490 from carbine barrel (13″)

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