556x45mm

The 5.56x45mm NATO, commonly written as 5.56x45mm or 556x45mm, is a rimless bottleneck rifle cartridge that has served as a standard NATO small-arms caliber since the 1960s. It was developed to replace older intermediate cartridges in service rifles and is widely used in weapons such as the M16 and M4 family as well as many allied forces’ rifles.

Design and dimensions: The cartridge uses a .224 inch (5.7 mm) diameter bullet seated in a relatively small, tapered, bottleneck case about 45 mm long. The overall cartridge length is typically around 57 mm. The case and chamber designs are optimized for higher pressure than civilian calibers, and the chamber geometry accommodates a range of bullet weights from light FMJ rounds to heavier hollow-point or penetrator designs.

Common loads and performance: Military-issue ammunition for the 5.56x45mm includes 55-grain full metal jacket (M193) and 62-grain armor-piercing or penetrator rounds (M855/SS109). M193 typically achieves muzzle velocities in the 3,000–3,250 feet per second range, depending on barrel length, while M855/SS109 generally runs around 2,900–3,100 feet per second. Bullet weights in civilian variants range from about 40 to 77 grains, with performance varying accordingly.

Interchangeability and civilian variants: The civilian counterpart is the .223 Remington, which has very similar dimensions but different maximum pressure standards and chamber tolerances. Firearms chambered for 5.56x45mm can typically fire .223 Remington ammunition, but the reverse is not always safe due to pressure and chamber differences. Civilian and military ammunition styles differ in construction and intended use.

Overall, the 5.56x45mm NATO remains a widely adopted, versatile cartridge core to modern military rifles, balancing light recoil with effective terminal performance for a range of engagements.