556×45

556×45mm NATO, commonly written as 5.56×45, is a rimless, bottleneck rifle cartridge designed for NATO service and used by many allied militaries. It was developed to provide lighter, higher-velocity ammunition for standard infantry rifles and has been paired with weapons such as the M16 and M4 family. The cartridge is typically chambered in rifles designed for NATO standards and has become one of the most widely used small-arms cartridges in the world.

Design and dimensions: The case length is 45 mm, with an overall cartridge length of about 57 mm. The bore diameter is around 0.224 inches (5.69 mm). It is a necked-down, rimless cartridge that accepts small rifle primers. Common military loads use lightweight bullets around 55 to 62 grains, driven at high velocities. The design supports a variety of bullet types, including full metal jacket, tracer, and armor-piercing designs.

Ammunition and variants: The standard service loads include the 55-grain M193 and the 62-grain M855, with the latter featuring a steel penetrator. Tracer variants such as M856 are used for fire adjustment and signaling, while armor-piercing variants such as M995 are designed for penetrating hard targets. In practice, multiple commercial and military loading options exist, adapted to specific weapons and mission requirements.

Performance and usage: Muzzle velocities for common 5.56×45 loads typically range from the high 2,900s to over 3,100 feet per second, depending on bullet weight and barrel length. Effective engagement ranges for standard infantry use are commonly cited around several hundred meters, with accuracy and performance influenced by rifle design, barrel twist rate, and ammunition type.

Chambering considerations: 5.56×45 ammunition is intended for rifles chambered for 5.56 NATO. Some rifles marked to fire both 5.56 and .223 Remington can safely do so, but rifles chambered strictly for .223 Remington may not be suitable for higher-pressure 5.56×45 loads. Users should follow manufacturer guidelines and local standards.

See also: .223 Remington, 7.62×51mm NATO.