5.56 × 45 mm

during development of the Armalite AR-15 rifle that was subsequently adopted by the US military as the M16. Original prototypes of the AR-15 were in 0.222 Remington calibre, but during testing in 1957, the army raised the ballistics requirements, necessitating a change in the cartridge. Eugene Stoner personally calculated the bullet weight and muzzle velocity necessary in order to achieve the army's requirements and decided that a slightly enlarged cartridge case was necessary, in addition to a heavier bullet. Stoner commissioned Remington to produce the cartridges with 3.56 g (55 grain) bullets, which were designated 0.222 Remington Special. The 0.222 Remington Special cartridge was officially adopted by the US Army in 1964 as the 5.56 × 45 mm M193. This cartridge was subsequently marketed commercially by Remington for sporting rifles as the 0.223 Remington. The 5.56 × 45 mm M193 cartridge was the standard US military rifle cartridge until introduction of the M855 ( SS109 ) cartridge in 1984. Because of the widespread use of the M16 and other rifles intended for use with the 3.56 g bullet, the M193 is still produced in large quantities worldwide. The M193 may be safely fired from rifles chambered for the SS109 cartridge. As a result of extensive tests which were carried out from 1976 to 1979, the 5.56 × 45 mm cartridge was standardised for NATO use, but with a heavier 4 g (62 grain) bullet designed by FN Herstal. This cartridge was designated SS109 in Europe and M855 when it was adopted by the US in 1984. The SS109 cartridge proved to actually outperform the 7.62 × 51 mm NATO cartridge in penetration tests and was more accurate than the M193. The new cartridge did, however require a faster (1:178 mm) rifling twist rate to stabilise its heavier bullet.