5.56 x 45 mm NATO, also known by Remington .223, is the size of standard rifle ammunition for NATO forces. 5.56 mm 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, 7.62 mm caliber. The cartridge is approximately 2.25 inches (57 mm) long and 0.38 inches (9.7 mm) in diameter. The bullet itself is 0.75 inches (20 mm) long and 0.25 inches (5.56 mm) in diameter.
The 5.56 NATO cartridge will penetrate approximately 15 to 20 inches (380 to 500 mm) 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 large wound profile generated by this cartridge at close ranges. This phenomenon of hydrostatic shock is at least fairly controversial, but seems to impart much greater damage to tissue than the wound track alone would suggest.
The NATO Ball round (the U.S. M855) can also penetrate up to 3 mm of steel while the Armor-piercing variant (the U.S. M995) can penetrate up to 6 mm.
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