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The 5.56×45mm NATO standard SS109/M855 cartridge was designed for maximum performance when fired from a 508 mm (20.0 in) long barrel, as was the original 5.56 mm M193 cartridge. Experiments with longer length barrels up to 610 mm (24.0 in) resulted in no improvement or a decrease in muzzle velocities for the SS109/M855 cartridge.
The current NATO 5.56mm SS109 (M855) bullet uses a steel-tipped lead core to improve penetration, the steel tip providing resistance to deformation for armor piercing, and the heavier lead core (25% heavier than the previous bullet, the M193) providing increased sectional density for better penetration in soft
The random nature of fragmentation required the military vest specification to trade off mass vs. ballistic-benefit. Hard vehicle armor is capable of stopping all fragments, but military personnel can only carry a limited amount of gear and equipment, so the weight of the vest is a limiting factor in vest fragment protection.
NATO chose a 178-mm (1-in-7) rifling twist rate for the 5.56×45mm NATO chambering. The SS109/M855 5.56×45mm NATO ball cartridge requires a 228 mm (1-in-9) twist rate, while adequately stabilizing the longer NATO L110/M856 5.56×45mm NATO tracer projectile requires an even faster 178 mm (1-in-7) twist rate. [8]
The M16A2 fires the improved 5.56×45mm (M855/SS109) cartridge and has a newer adjustable rear sight, case deflector, heavy barrel, improved handguard, pistol grip, and buttstock, as well as a semi-auto and three-round burst fire selector. [21] Adopted in July 1997, the M16A4 is the fourth generation of the M16 series.
Type 89 rifle ammunition is interchangeable with the 5.56×45mm SS109/M855 round used by the U.S. military and NATO. [17] Along with the 7.62×51mm round used in the Type 64 rifle, this allows for interchangeability with ammunition stockpiles of U.S. forces stationed in Japan.
The AUG is chambered for the 5.56×45mm NATO cartridge and has the standard 1:9 rifling twist that will stabilise both SS109/M855 and M193 rounds. Some nations including Australia, Ireland and New Zealand use a version with a 1:7 twist optimised for the SS109 NATO round. The submachine gun variants are chambered in either 9×19mm Parabellum or ...
5.56×45mm SS109: NATO standard 5.56×45mm cartridge. [17] 5.7×28mm: Small-caliber, high-velocity cartridge designed for use with the FN P90 PDW and FN Five-seven pistol. [18] EGLM: 40mm Ergonomic Grenade Launcher Module designed for the FN SCAR. 303: Less-lethal 17 mm multi-shot projectile launcher. [19]