5.56×45mm NATO is a rifle

Rifle

A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...

 cartridge

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

 developed in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and originally chambered in the M16 rifle

M16 rifle

The M16 is the United States military designation for the AR-15 rifle adapted for both semi-automatic and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO...

. Under STANAG

STANAG

STANAG is the NATO abbreviation for Standardization Agreement, which sets up processes, procedures, terms, and conditions for common military or technical procedures or equipment between the member countries of the alliance. Each NATO state ratifies a STANAG and implements it within their own...

 4172, it is a standard cartridge for NATO forces as well as many non-NATO countries. It is derived from, but not identical to, the .223 Remington

.223 Remington

The .223 Remington is a sporting cartridge with almost the same external dimensions as the 5.56×45mm NATO military cartridge. The name is commonly pronounced either two-two-three or two-twenty-three. It is loaded with a diameter, jacketed bullet, with weights ranging from , though the most common...

 cartridge. When the bullet impacts at high velocity and yaw

Yaw

The word yaw can refer to:* Yaw angle, one of the Tait-Bryan angles, describing the heading of a vehicle or machine, and some other related elements:**Yaw system, component responsible for the orientation of a wind turbine towards the wind....

s in tissue, fragmentation creates a rapid transfer of energy which can result in dramatic wounding effects.
The previous standard NATO rifle cartridge was the 7.62×51mm NATO

7.62×51mm NATO

The 7.62×51mm NATO is a rifle cartridge developed in the 1950s as a standard for small arms among NATO countries...

, derived from the .308 Winchester

.308 Winchester

The .308 Winchester is a rifle cartridge and is the commercial cartridge upon which the military 7.62x51mm NATO centerfire cartridge is based. The .308 Winchester was introduced in 1952, two years prior to the NATO adoption of the 7.62x51mm NATO T65...

 rifle cartridge and designed to replace the U.S.