5.56mm caliber, also known by .223 caliber is the size of standard rifle ammunition for NATO forces. 5.56mm amunition was introduced for use in the American M16 rifle in the 1960s. It was used as an alternative to the previous NATO standard ammunition size, 7.62mm caliber.

The 5.56 NATO cartridge will penetrate approximately 15-20 inches in soft tissue, assuming it is a standard military ball projectile being fired; these cartridges, at close range, propel the bullet fast enough so that it yaws early on in penetrating tissue, causing the bullet to fracture and fragment at the cannelure. This is due in part to the rotational energy imparted by the high rate of twist in newer M16 barrels (A1 and later models), and is responsible for the respectable wound profile generated by this cartridge at close ranges.

Weapons using 5.56mm ammunition include