5.56x45mm NATO, standardized under STANAG 4172, is a riflecartridge. It is a standard cartridge for NATO forces, and for several nations not part of NATO. It is derived from, but not entirely interchangeable with, the .223 Remington cartridge. This is due to, among other things, the greater gas pressure of the 5.56 mm military round.
The previous standard NATO rifle round was the 7.62x51mm NATO, derived from the .300 Savage 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 7.62 mm round 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 5.56 mm ammunition as 7.62 mm 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" round since 1945 and were on the point of introducing a .280 inch (7 mm) round when the selection of the 7.62 mm round 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 round. 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 7.62 mm NATO. Of the cartridges tendered, the 5.56 mm was successful, but not the 5.56 mm 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 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 rounds lose effectiveness at much shorter ranges than longer-barreled rifles.
There has been much criticism of the poor performance of the round, especially the first-round 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 round is reported to be mostly effective, and its tendency to fragment reduces the risk of "overpenetration" when used at close range. However, if the round is moving too slowly to reliably fragment on impact, the wound size and potential to incapacitate a target is greatly reduced. Several alternate cartridges have been developed in an attempt to address the perceived shortcomings of 5.56 mm ammunition including the 6.5 Grendel and the 6.8 SPC.
Recently, advances have been made in 5.56 mm 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 round to fire from their M4A1 carbines.
While the 5.56 mm and .223 cartridges are very similar, they are not identical. Military cases are made from thicker brass than commercial cases, which reduces the powder capacity (an important consideration for handloaders), and the NATO specification allows a higher chamber pressure. Test barrels made for 5.56 mm NATO measure chamber pressure at a the case mouth, as opposed to the SAAMI location. This difference accounts for upwards of 20,000 psi (140 MPa) difference in pressure measurements. That means that advertised pressure of 58,000 psi (400 MPa) for 5.56 mm NATO, is around 78,000 psi (540 MPa) tested in .223 Rem test barrels. The 5.56 mm chambering, known as a NATO or mil-spec chambers, have 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 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 5.56 mm and .223 equally well.
Using commercial .223 cartridges in a 5.56-chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223-chambered gun due to the excessive lead. Using 5.56 mil-spec cartridges (such as the M855) in a .223-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 5.56 mm, 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 5.56 mm ammunition.
The NATO Ball round (U.S.: M855) can penetrate up to 3 mm (about 1/8") of steel at 600 meters. According to Nammo, a Norwegian ammunition producer, the M995 can penetrate up to 12 mm (nearly 1/2") of RHA steel at 100 meters. 
Cartridge, Caliber 5.56 mm, Armor Piercing, M995(United States): 5.56x45mm 52-grain AP cartridge, black cartridge tip.
Cartridge, Caliber 5.56 mm, Tracer, XM996(United States): 5.56x45mm so-called "Dim Tracer" with reduced effect primarily for use with night vision devices.
Cartridge, Caliber 5.56 mm, Special Ball, Long Range, Mk 262 Mod 0/1(United States): 5.56x45mm 77-grain Open-Tipped Match/Hollow-Point Boat-Tail cartridge. Mod 0 features Sierra Matchking bullet, while Mod 1 features either Nosler or Sierra bullet.
Cartridge, 5.64 mm, Ball, MLU-26/P(United States): Early USAF designation for 5.56x45mm ball cartridge produced by Remington.