556×45mm

The 5.56x45mm NATO is a rifle cartridge developed in the late 1950s and adopted by the U.S. Army in 1964. It was designed as a smaller, lighter, and higher-velocity round than the .30-06 Springfield and 7.62x51mm NATO cartridges. The goal was to allow soldiers to carry more ammunition and achieve better control during automatic fire.

The cartridge is based on the .223 Remington cartridge, which was developed by Remington Arms. The NATO version differs slightly in its exact specifications to ensure interoperability among member nations. The 5.56x45mm NATO is a bottlenecked, rimless case firing a spitzer boat-tail projectile. Common bullet weights range from 55 to 77 grains.

This cartridge is widely used by military forces around the world, most notably in the M16 rifle and its derivatives, as well as the FN SCAR, H&K G36, and many others. Its high velocity contributes to good accuracy and terminal ballistics, though its effectiveness against lightly armored targets can be limited compared to larger caliber rounds. The round's relatively low recoil also makes it suitable for automatic fire. Over the years, various bullet constructions have been developed to improve its performance in different scenarios.