5.56×45mm

5.56×45mm NATO
5.56×45mm NATO with measurement, left to right: projectile, empty case, complete round with projectile in case
Type	Rifle
Place of origin	United States
Service history
In service	1963–present
Used by	NATO
Wars	Since Vietnam War
Production history
Designer	Eugene Stoner
Specifications
Parent case	.223 Remington
Case type	Rimless, bottleneck
Bullet diameter	5.70 mm (0.224 in)
Neck diameter	6.43 mm (0.253 in)
Shoulder diameter	9.00 mm (0.354 in)
Base diameter	9.58 mm (0.377 in)
Rim diameter	9.60 mm (0.378 in)
Rim thickness	1.14 mm (0.045 in)
Case length	44.70 mm (1.760 in)
Overall length	57.40 mm (2.260 in)
Case capacity	1.85 cm3 (28.5 gr H2O)
Rifling twist	178 mm or 229 mm (1 in 7 in or 9 in, originally 1 in 14 in)
Primer type	Small rifle
Maximum pressure (EPVAT)	430.00 MPa (62,366 psi)
Maximum pressure (SCATP 5.56)	380.00 MPa (55,114 psi)
Ballistic performance
Bullet mass/type Velocity Energy 4 g (62 gr) SS109 FMJBT 940 m/s (3,100 ft/s) 1,767 J (1,303 ft·lbf) 4.1 g (63 gr) DM11 FMJBT 936 m/s (3,070 ft/s) 1,796 J (1,325 ft·lbf) 4.1 g (63 gr) GP 90 FMJBT 905 m/s (2,970 ft/s) 1,679 J (1,238 ft·lbf)
Test barrel length: 508 mm (20.0 in) Source(s): NATO EPVAT testing, QuickLOAD, SAAMI, C.I.P.

The 5.56×45mm NATO (official NATO nomenclature 5.56 NATO) is an intermediate cartridge developed in the United States and originally chambered in the M16 rifle. Under STANAG 4172, it is a standard cartridge for NATO forces as well as many non-NATO countries. It is derived from, but is not identical to, the .223 Remington cartridge.

In the 1950s, the 7.62×51mm NATO rifle cartridge (physically interchangeable with, but not identical to, the .308 Winchester rifle cartridge) was selected to replace the .30-06 Springfield as the standard NATO rifle cartridge. At the time of selection, there had been criticism that the 7.62×51mm NATO was too powerful for lightweight modern service rifles, causing excessive recoil, and that as a result it did not allow for sufficient automatic rate of fire from hand-held weapons in modern combat.

The British had extensive evidence with their own experimentation with intermediate cartridges since 1945 and were on the point of adopting a .280 inch (7 mm) cartridge when the selection of the 7.62×51mm NATO was made. The FN company had also been involved in the development of the .280 round including developing a version of the FN FAL in .280. The concerns about recoil and effectiveness were effectively overruled by the US within NATO, and the other NATO nations accepted that standardization was more important at the time than selection of the ideal cartridge. However, while the 7.62×51mm NATO round became NATO standard, the US was already engaged in research of their own, which ultimately led to the 5.56×45mm NATO cartridge.

During the late 1950s, ArmaLite and other U.S. firearm designers started their individual Small Caliber/High Velocity (SCHV) rifle experiments using the commercial .222 Remington cartridge. When it became clear that there was not enough case capacity to meet U.S. Continental Army Command's (CONARC) velocity and penetration requirements, ArmaLite contacted Remington to create a similar cartridge with a longer case body and shorter neck. This became the .222 Special. At the same time, Springfield Armory's Earle Harvey had Remington create an even longer cartridge case then known as the .224 Springfield. Springfield was forced to drop out of the CONARC competition, and thus the .224 Springfield was released in 1958 as a commercial sporting cartridge known as the .222 Remington Magnum. To prevent confusion among all of the competing .222 cartridge designations, the .222 Special was renamed the .223 Remington in 1959. In the spring of 1962 Remington submitted the specifications of the .223 Remington to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI). With the U.S. military adoption of the M16 assault rifle in 1963, the .223 Remington in a slightly derived form was standardized as the 5.56×45mm NATO. As a commercial sporting cartridge the .223 Remington was introduced in 1964.