5.56x45mm NATO is a rifle 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 not identical to, the .223 Remington cartridge.
The previous standard NATO rifle cartridge was the 7.62x51mm NATO, derived from the .308 Winchester rifle cartridge and designed to replace the U.S. military's .30-06 Springfield rifle cartridge. At the time of selection, there had been criticism that the 7.62mm was too powerful for modern service rifles, causing excessive recoil, and that the weight of the ammunition did not allow for enough "firepower" in modern combat. A soldier can carry nearly twice as much 5.56mm ammunition as 7.62mm for the same weight.
During the late 1950s, ArmaLite and other U.S. firearm designers started their individual Small Caliber/High Velocity (SCHV) assault rifle experiments using the commercial .222 Remington cartridge. When it became clear that there was not enough powder 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 Remington 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 later released as a commercial sporting cartridge known as the .222 Remington Magnum. To prevent confusion with all of the competing .222 cartridge designations, the .222 Remington Special was renamed the .223 Remington. After playing with their own proprietary cartridge case design, the .224E1 Winchester, Winchester eventually standardized their case dimensions, but not overall loaded length, with the .222 Remington Special to create a cartridge known as the .224E2 Winchester. With the U.S. military adoption of the ArmaLite AR-15 as the M16 rifle in 1963, the .223 Remington was standardized as the 5.56x45mm. However, the .223 Remington was not introduced as a commercial sporting cartridge until 1964.
The British had extensive evidence with their own experiments into an "intermediate" cartridge since 1945 and were on the point of introducing a .280 inch (7 mm) cartridge when the selection of the 7.62mm NATO was made. The FN company had also been involved. 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 the concerns would prove to be valid and led to the development of the 5.56 cartridge.
During the 1970s, NATO members signed an agreement to select a second, smaller caliber cartridge to replace the 7.62mm NATO. Of the cartridges tendered, the 5.56mm was successful, but not the 5.56mm loading (M193 Ball) as used by the U.S. at that time. Instead, the Belgian FN SS109 loading was chosen for standardization. The SS109 used a heavier bullet with a steel core inserted, fired at a lower muzzle velocity for better long-range performance, specifically to meet a requirement that the bullet be able to penetrate through one side of a steel helmet at 600 m. Some believe that this requirement has made the M855 less capable of fragmentation than the M193 as discussed below.
The 5.56x45mm NATO cartridge with the standard military ball bullet (NATO: SS109; U.S.: M855) will penetrate approximately 15 to 20 inches (38 to 50 cm) into soft tissue in ideal circumstances. As with all spitzer shaped projectiles it is prone to yaw in soft tissue. However, at impact velocities above roughly 2,700 ft/s (820 m/s), it may yaw and then fragment at the cannelure (the groove around the cylinder of the bullet). These fragments can disperse through flesh and bone, inflicting additional internal injuries. Fragmentation, if and when it occurs, seems to impart much greater damage to tissue than bullet dimensions and velocities would suggest. This fragmentation effect is highly dependent on velocity, and therefore barrel length: short-barreled rifles generate less muzzle velocity and therefore lose effectiveness at much shorter ranges than longer-barreled rifles.
There has been much criticism of the poor performance of the bullet on target, especially the first-shot kill rate when using firearms that don't achieve the velocity to cause fragmentation. This typically becomes an issue at longer ranges (over 100 m) or when penetrating heavy clothing, but this problem is compounded in shorter-barreled weapons. The 14.5-inch (37 cm) barrel of the U.S. military's M4 Carbine can be particularly prone to this problem. At short ranges, the bullet is reported to be mostly effective, and its tendency to fragment reduces the risk of "overpenetration" when used at close range. However, if the bullet is moving too slowly to reliably fragment on impact, the wound size and potential to incapacitate a person is greatly reduced. Several alternate cartridges have been developed in an attempt to address the perceived shortcomings of 5.56mm ammunition including the 6.5 Grendel and the 6.8 SPC.
Recently, advances have been made in 5.56mm ammunition. The US military has adopted for limited issue a 77-grain (5.0 g) "Match" bullet, type classified as the Mk 262. The heavy, lightly constructed bullet fragments more violently at short range and also has a longer fragmentation range. Originally designed for use in the Mk 12 SPR, the ammunition has found favor with special forces units who were seeking a more effective cartridge to fire from their M4A1 carbines.
While the 5.56mm NATO and .223 Remington cartridges and chamberings are very similar, they are not identical.
Military cases are generally made from thicker brass than commercial cases; this reduces the powder capacity (an important consideration for handloaders), and the NATO specification allows a higher chamber pressure. NATO EPVAT test barrels made for 5.56mm NATO measure chamber pressure at the case mouth, as opposed to the location used by the United States civil standards organization SAAMI. The piezoelectric sensors or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ. This difference in measurement method accounts for upwards of 137.9 MPa (20,000 psi) difference in pressure measurements. This means the NATO EPVAT maximum service pressure of 430 MPa (62,366 psi) for 5.56mm NATO, is reduced by SAAMI to 379.21 MPa (55,000 psi) for .223 Remington. In contrast to SAAMI, the other main civil standards organization C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56mm NATO.
The 5.56mm NATO chambering, known as a NATO or mil-spec chamber, has a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 Remington chambering, known as SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber (Rock River Arms) or the Armalite chamber, which are designed to handle both 5.56mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56mm NATO chamber specification.
Using commercial .223 Remington cartridges in a 5.56mm NATO chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223 Remington chambered gun due to the longer lead. Using 5.56mm NATO mil-spec cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice. Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56mm NATO ammunition.
Comparison of 5.56mm versus 7.62mm NATO
Cartridge Cartridge size Bullet weight Velocity Energy
5.56mm NATO 5.56x45mm 3.95–5.18 g 772–930 m/s (2526–3051 ft/s) 1,177–2,240 J
7.62mm NATO 7.62x51mm 9.33 g 838 m/s (2749 ft/s) 3,275 J