5.56x45mm NATO with measurement, left to right: bullet, empty case, complete round with bullet in casing.
The 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. When the bullet impacts at high velocity and yaws in tissue, fragmentation creates a rapid transfer of energy which can result in dramatic wounding effect.
History[]
5.56x45mm NATO with measurement, left to right: bullet, emptly case, complete round with bullet in casing.
The previous standard NATO rifle cartridge was the 7.62x51mm NATO, derived from the .308 Winchester rifle cartridge and designed to replace the .30-06 Springfield cartridge in the US military. At the time of selection, there had been criticism that the 7.62 was too powerful for modern service rifles, causing excessive recoil, and that the weight of the ammunition did not allow for enough rate of fire in modern combat.
The British had extensive evidence with their own experiments into an intermediate cartridge since 1945 and were on the point of introducing a .280 cal cartridge when the selection of the 7.62x51mm NATO was made. However, whilst the 7.62x51mm NATO round became NATO standard the US was already engaged in research of their own, which ultimately led to the 5.56x45mm NATO cartridge.
During the late 1950s, ArmaLite and other US 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 US 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 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 .224 E2 Winchester. With the US military adoption of the ArmaLite Ar-15 as the M16 rifle in 1963, the .223 Remington was standardized as the 5.56x45mm NATO. as a commercial sporting cartridge the .223 Remington was only introduced in 1964.
In a series of mock-combat situations testing in the early 1960s with the AR-15, M14, and AK-47, the Army found that the AR-15's small size and light weight allowed it to be brought to bear more quickly. Their final conclusion was that an 8-man team armed with the AR-15 would have the same fire-power as a current 11-man team armed with the M14. US troops were able to carry more than twice as much 5.56x45mm NATO ammunition as 7.62x51mm NATO for the same weight, which would allow them a better advantage against a typical NVA unit armed with AK-47s.
| Rifle | Cartridge | Cartridge weight | Weight of loaded magazine | Max. 10 kilogram ammo. load |
|---|---|---|---|---|
| M14 | 7.62×51mm | 393 gr (25.4 g) | 20 rd mag @ 0.68 kg | 14 mags @ 9.52 kg for 280 rds |
| M16 | 5.56×45mm | 183 gr (11.8 g) | 20 rd mag @ 0.3 kg | 33 mags @ 9.9 kg for 660 rds |
| AK-47 | 7.62×39mm | 281 gr (18.2 g) | 30 rd mag @ 0.92 kg
|
10 mags @ 9.2 kg for 300 rds |
(*AK-47 magazines are much heavier that M14 and M16 magazines)
Service rifle cartridge cases: (Left to right) 7.62x54mmR, 7.62x51mm NATO, 7.62x39mm, 5.56x45mm NATO, 5.45x39mm.
In 1977, NATO members signed an agreement to select a second, smaller caliber cartridge to replace the 7.62x51mm NATO cartridge. Of the cartridges tendered, the 5.56x45mm NATO was successful, but not the 55 gr M193 round used by the US at that time. The wounds produced by the M193 round were so devastating that many considered it to be inhumane. Instead, the Belgian 62 gr S109 round was chosed for standardization. The SS109 used a heavier bullet with a steel core and had 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 meters. This requirement made the SS109 (M855) round less capable of fragmentation than the M193 and was considered more humane.
The 5.56x45mm NATO inspired an international tendency toward relatively small sized, light weight, high velocity military service cartridges that produce relatively low bolt thrust and free recoil impulse, favoring light weight arms design and automatic fire accuracy. Similar intermediate cartridges were developed and introduced by the Soviet Union in 1974 (5.45x39mm) and by the People's Republic of China (5.8x42mm).
Cartridge dimensions[]
The 5.56x45mm NATO has 1.85 ml cartridge case capacity. Americans would define the shoulder angle at alpha/2 = 23 degrees. The common rifling twist rate for this cartridge is 178 mm or 229 mm, 6 grooves, and the primer type is small rifle.
According to the official NATO proofing guidelines the 5.56x45mm NATO case can handle up to 430 MPa piezo service pressure. In NATO regulated organizations every rifle cartridge combo has to be proofed at 125% of this maximum pressure to certify for service issue. This is equal to the C.I.P. maximum pressure guideline for the .223 Remington cartridge, that is, the 5.56x45mm NATO parent cartridge.
Performance[]
5.56x45mm NATO shown alongside other cartridges.
The 5.56x45mm NATO cartridge with the standard 62 gr steel core bullets will penetrate approximately 15 to 20 in 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 2,500 ft/s, it may yaw and then fragment at teh cannelure (the crimping groove around the cylinder of the bullet). These fragments can disperse through flesh and boe, inflicting additional internal injuries.
Fragmentation, if or when it occurs, imparts much greater damage to human tissue than bullet dimensions and velocities would suggest. This fragmentation effect is highly dependent on velocity, and therefore, barrel length: short-barreled carbines generate less muzzle velocity and therefore lose wounding effectiveness at much shorter ranges than longer barreled rifles. Proponents of the hydrostatic shock theory contend that the rapid transfer of energy also results in wounding effects beyond the tissue directly crushed and torn by the bullet and fragments. These remote wounding effects are known as hydrostatic shock.
NATO Ball (US: M855) can penetrate up to 3 mm of steel at 600 meters. According to Nammo, a Norwegian ammunition producer, the M995 can penetrate up to 12 mm of RHA steel at 100 m.
The US Army's Ballistic Research Laboratory measured a ballistic coefficient of 0.151 and form factor of 1.172 of the SS109/M855 ball projectile.
Criticism[]
.jpg){kind=link}
Loading 5.56x45mm NATO cartridges into a STANAG magazine using a stripper clip.
There has been much critcism of the poor performance of the bullet on target, especially the first-shot kill rate when the muzzle velocity of the firearms used and the downrange bullet decceleration do not achieve the minimally required terminal velocity at the target to cause fragmentation. This wounding problem has been cited in incidents beginning in the First Gulf War, Somalia, and in the current conflicts in Iraq and Afghanistan. In recent lab testing of M855, it has been shown that the bullets do not fragment reliably or consistently from round-to-round, displaying widely variable performance. In several cases, yawing did not begin
5.56x45mm NATO in a STANAG magazine.
until 7-10 inches of penetration. This was with all rounds coming from the same manufacturer. This lack of wounding capacity typically becomes an increasingly significant issue as range increases (e.g., ranges over 50 m when using an M4 or 200 m when using an M16) or when penetrating heavy clothing, but this problem is compounded in shorter-barreled weapons. the 14.5 in barrel of the US military's M4 carbine generates considerably less inital velocity than the longer 20" barrel found on the M16, and terminal performance can be a particular problem with the M4.
- Combat operations the past few months have again highlited terminal performance deficiencies with 5.56x45mm 62 gr. M855 FMJ. These problems have primarily been manifested as inadequate incapacitation of enemy forces despite them being hit multiple times by M855 bullets. These failures appear to be associated with the bullets exiting the body of the enemy soldier without yawing or fragmenting.
- This failure to yaw and fragment can be caused by reduced impact velocities as when fired from short barrel weapons or when the range increases. It can also occur when the bullets pass through only minimal tissue, such as a limb or the torso of a thin, small-statured individual, as the bullet may exit the body before it has a chance to yaw and fragment. In addition, bullets of the SS109/M855 type are manufactured by many countries in numerous production plants.
- Although all SS109/M855 types must be 62 gr. FMJ bullets constructed with a steel penetrator in the nose, the composition, thickness, and relative weights of the jackets, penetrators, and cores are quite variable, as are the types and position of the cannelures. Because of the significant differences in construction between bullets within the SS109/M855 category, terminal performance is quite variable-with differences noted in yaw, fragmentation, and penetration depths.
Despite complaints that the 5.56 round lacks stopping power, others contend that animal studies of the wounding effects of the 5.56x45mm round versus the 7.62x39mm have found that the 5.56mm round is more damaging, due to the post-impact behavior of the 5.56mm projectile resulting in greater cavitation of soft tissues. The US Army contended in 2003 that the lack of close range lethality of the 5.56x45mm was more of a perception than fact. With controlled pairs and good shot placement to the head and chest, the target was usually defeated without issue. The majority of failures were the result of hitting the target in non-vital areas such as extremities. However, a minority of failures occurred in spite of multiple hits to the chest.
Improvements[]
Recently, advances have been made in 5.56mm ammunition. The US military has adopted for limited issue a 77-gr "Match" bullet, type classified as the Mk262. 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. Commerically available loadings using these heavier (and longer) bullets can be prohibitively expensive and cost much more than military surplus ammunition.
For general issue, the US military adopted the M855A1 round in 2010 to replace the M855. The primary reason was pressure to use non-lead bullets. The bullet uses a copper alloy slug with a steel penetrator, reducing lead contamination to the environment. The round can better penetrate brick, concrete, and masonry walls, as well as body armor and sheet metal. The propellant burns faster which decreases the muzzle flash, an important feature when fired from a short barreled M4 carbine. Though the M855A1 offers several improvements other than being lead-free. It is slightly more accurate and has an increased penetrating capability. The round can better penetrate brick, concrete, and masonry walls, as well as body armor and sheet metal. The propellant burns faster which decreases the muzzle flash, an important feature when fired from a short-barreled M4 carbine. Though the M855A1 is more expensive to produce, its increased performance compensates. One possible danger is that it generates more pressure in the chamber when fired, slightly increasing the risk of catastrophic failure of the weapon, though this has yet to occur. From fielding in June 2010 to September 2012, Alliant Techsystems has delivered over 350 million M855A1 Enhanced Performance Rounds.
Alternatives[]
If the 5.56mm bullet is moving too slowly to reliably fragment on impact, the wound size and potential to incapacitate a person is greatly reduced. There have been numerous attempts to create an intermediate cartridge that addresses the complaints of 5.56mm NATO's lack of stopping power along with lack of controllability seen in rifles firing 7.62 NATO in full auto. Other cartridges focused on superior short-range performance by sacrificing long-distance performance due to relatively short engagement distances typically observed in modern warfare. As of late 2009, none of those cartridges gained any significant traction beyond special forces and sport shooting communities. Texamples include, but are not limited to, the 6.8mm Remington SPC and 6.5mm Grendel.
5.56mm NATO versus .223 Remington[]
The 5.56mm NATO and .223 Remington cartridges and chamberings are similar but not identical. Military 5.56x45mm cases are often made thicker and therefore have less case capacity. However, the NATO specification allows a higher chamber pressure. NATO EPVAT test barrels made for 5.56 NATO measure chamber pressure at the case mouth, as opposed to the location used by the US civil standards organization SAAMI. The piezioelectric sensors or transducers NATO and SAAMI use to conduct the actual pressure measurements also differ. This difference in measurement method accounds for upwards of 20,000 psi difference in pressure measurements. This means the NATO EPVAT maximum service pressure of 430 MPa for 5.56mm NATO, is reduced by SAAMI to 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 5.56x45mm 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 tested at the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber or the ArmaLite chamber, which are designed to handle both 5.56x45mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56x45mm NATO chamber specification.
Using commercial .223 Remington cartridges in a 5.56x45mm NATO chambered rifle should work reliably, but generally will not be as accuracte as when fired from a .223 Remington chambered gun due to the longer leade. Using 5.56x45mm 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 SAAMI recommends against the practice. Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56x45mm NATO, such as many commerical AR-15 variants and the Ruger Mini-14 (marked ".223 cal"), but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and sighs of excessive pressures (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56x45mm NATO ammunition.
It should be noted that the upper receiver (to which the barrel with its chamber are attached) and the lower receiver are entirely seperate parts in AR-15 style rifles. If the lower receiver has either .223 or 5.56 stamped on it, it does not guarantee the upper assembly is rated for the same caliber, because the upper and lower receiver in the same rifle can, and frequently do, come from different manufacturers-particularly with rifles sold to civilians or second-hand rifles.
In more practical terms, as of 2010 most AR-15 parts suppliers engineer their complete upper assemblies to support both calibers in order to protect their customers from injuries and to protect their businesses from resultant litigation.