5 . 56×45mm NATO The 5 . 56×45mm NATO ( official NATO nomenclature 5 . 56 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 . If the bullet impacts at high enough velocity and yaws in tissue , fragmentation creates a rapid transfer of energy which can result in dramatic wounding effects . History . The previous standard NATO rifle cartridge was the 7 . 62×51mm NATO , sold commercially as the . 308 Winchester rifle cartridge , and designed to replace the . 30 - 06 Springfield rifle cartridge in the U . S . military . At the time of selection , there had been criticism that the 7 . 62×51mm NATO was too powerful for light weight modern service rifle s , 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 inch ( 7 mm ) cartridge when the selection of the 7 . 62×51mm 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 , whilst 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 ) 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 M16 rifle in 1963 , the . 223 Remington was standardized as the 5 . 56×45mm NATO . As a commercial sporting cartridge the . 223 Remington was only introduced in 1964 . The 5 . 56×45mm cartridge , along the M16 rifle , were initially adopted by U . S . infantry forces as interim solutions to address the weight and control issues experienced with the 7 . 62×51mm round and M14 rifle . In the late 1950s , the Special Purpose Individual Weapon program sought to create flechette rounds to allow troops to fire sabot - type projectiles to give a short flight time and flat trajectory with a muzzle velocity of to . At those speeds , factors like range , wind drift , and target movement would no longer affect performance . Several manufacturers produced varying weapons designs , including traditional wooden , bullpup , " space age , " and even multi - barrel designs with drum magazines . All used similar ammunition firing a 1 . 8 mm diameter dart with a plastic " puller " sabot filling the case mouth . While the flechette ammo had excellent armor penetration , there were doubts about their terminal effectiveness against unprotected targets . Conventional cased ammunition was more accurate and the sabots were expensive to produce . The SPIW never created a weapons system that was combat effective , so the M16 was retained , and the 5 . 56 mm round was kept as the standard U . S . infantry rifle cartridge . In a series of mock - combat situations testing in the early 1960s with the M16 , M14 and AK - 47 , the Army found that the M16 ' s small size and light weight allowed it to be brought to bear much more quickly . Their final conclusion was that an 8 - man team equipped with the M16 would have the same fire - power as a current 11 - man team armed with the M14 . U . S . troops were able to carry more than twice as much 5 . 56×45mm NATO ammunition as 7 . 62×51mm NATO for the same weight , which would allow them a better advantage against a typical NVA unit armed with AK - 47s . In 1977 , NATO members signed an agreement to select a second , smaller caliber cartridge to replace the 7 . 62×51mm NATO cartridge . Of the cartridges tendered , the 5 . 56×45mm NATO was successful , but not the 55 gr M193 round used by the U . S . at that time . The wounds produced by the M193 round were so devastating that many consider it to be inhumane . Instead , the Belgian 62 gr SS109 round was chosen 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 . 56×45mm NATO inspired an international tendency towards 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 . 45×39mm ) and by the People ' s Republic of China in 1987 ( 5 . 8×42mm ) . Cartridge dimensions . The 5 . 56×45mm NATO has 1 . 85 ml ( 28 . 5 grains HO ) cartridge case capacity . 5 . 56×45mm NATO maximum NATO cartridge dimensions . All sizes in millimeters ( mm ) . Americans would define the shoulder angle at alpha / 2 = 23 degrees . The common rifling twist rate for this cartridge is 178 mm ( 1 in 7 in ) or 229 mm ( 1 in 9 in ) , 6 grooves , Ø lands = 5 . 56 mm , Ø grooves = 5 . 69 mm , land width = 1 . 88 mm and the primer type is small rifle . According to the official NATO proofing guidelines the 5 . 56×45mm NATO case can handle up to 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 . 56×45mm NATO parent cartridge . Performance . The 5 . 56×45mm NATO cartridge with the standard 62 gr . steel core bullets ( NATO : SS109 ; U . S . : M855 ) will penetrate approximately 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 , it may yaw and then fragment at the cannelure ( the crimping groove around the cylinder of the bullet ) . These fragments can disperse through flesh and bone , 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 ( U . S . : M855 ) can penetrate up to 3 mm ( about in ) of steel at 600 meters . According to Nammo , a Norwegian ammunition producer , the M995 can penetrate up to 12 mm ( nearly in ) of RHA steel at 100 meters . The US Army ' s Ballistic Research Laboratory measured a ballistic coefficient ( G7 BC ) of 0 . 151 and form factor ( G7 " i " ) of 1 . 172 for the SS109 / M855 ball projectile . The Swedish military has measured the bullet velocities of SS109 / M855 military cartridges at from the muzzle fired from differing barrel lengths : Criticism . There has been much criticism of the allegedly 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 deceleration 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 Vietnam War , first Gulf War , Somalia , and in the conflicts in Iraq and Afghanistan . In 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 until 7 – 10 in 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 barrel of the U . S . military ' s M4 carbine generates considerably less initial velocity than the longer 20 " barrel found on the M16 , and terminal performance can be a particular problem with the M4 . Despite complaints that the 5 . 56 round lacks stopping power , others contend that animal studies of the wounding effects of the 5 . 56×45mm round versus the 7 . 62×39mm have found that the 5 . 56 mm round is more damaging , due to the post - impact behavior of the 5 . 56 mm projectile resulting in greater cavitation of soft tissues . The US Army contended in 2003 that the lack of close range lethality of the 5 . 56×45mm was more a matter of 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 . Some have contended that shot placement is the most important parameter in determining the lethality of a bullet . Shots to the central nervous system and major blood vessels are the primary forms of incapacitation . Difficulty at range can be attributed to training , as only a few nations taught shooting beyond 200 – 300 meters to regular soldiers . At a marksmanship training workshop held in Spain in March 2010 , some participants went from using 150 rounds over 28 hours to engage targets out to 200 meters , to using 1 , 000 rounds over two weeks engaging targets out to 400 meters . Sweden and Canada had requirements to shoot 3 - 5 rounds , respectively , in the prone or kneeling position at 100 meters with a max dispersion of . Due to their inability to adjust rifle fire for dispersion , target movement , unknown ranges , and wind drift , Swedish ISAF units relied on . 50 BMG heavy machine guns for long - range . The effect of barrel length is also questioned . A barrel length reduction of 59 percent results in a reduced muzzle velocity for 5 . 56 NATO ball of only 21 percent . The length differences between a 20 in M16 barrel and 14 . 5 in M4 barrel results in a trajectory difference of at 250 m . A bullet fired from an M16 has a greater striking velocity than one from an M4 only up to 50 meters longer distance . Time of flight difference from the two barrel lengths is per meter per second at 300 meters . Underperformance can be attributed to errors in range estimation , target lead , wind , firing position , and stress under fire , which are factors that can be resolved through training . Improvements . Advances have been made in 5 . 56 mm ammunition . The U . S . Military had 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 . Commercially available loadings using these heavier ( and longer ) bullets can be prohibitively expensive and cost much more than military surplus ammunition . Additionally , these heavy - for - caliber loadings sacrifice even more penetrative ability than the M855 round ( which has a steel penetrator tip ) . Performance of 5 . 56×45 mm military ammunition can generally be categorized as almost entirely dependent upon velocity in order to wound effectively . Heavy OTM bullets enhance soft tissue wounding ability at the expense of hard - target / barrier penetration . U . S . Special Forces had sought to create a round that had increased power out of carbine M4 barrels and compact SCAR - L barrels , while increasing hard target performance . Developmental efforts led to the creation of the Mk318 . The bullet uses an open - tip design to inflict damage on soft tissue , and has a brass rear to penetrate hard targets . The tip and lead core fragments consistently even out of short barrels , while the rear moves through once the front impacts . For general issue , the U . S . Army adopted the M855A1 round in 2010 to replace the M855 . The primary reason was pressure to use non - lead bullets . The lead slug is replaced by a copper alloy slug in a reverse - drawn jacket , with a hardened steel penetrator extending beyond the jacket , reducing lead contamination to the environment . The M855A1 offers several improvements other than being lead - free . It is slightly more accurate , has better consistency of effect in regards to wounding ability , and has an increased penetrating capability . The round can better penetrate steel , brick , concrete , and masonry walls , as well as body armor and sheet metal . It penetrates of mild steel at 350 meters , which the M855 can only do at 160 meters . The propellant burns faster , which decreases the muzzle flash and gives a higher muzzle velocity , 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 . The M855A1 bullet has a greater length than the M855 . Because steel and copper are less dense than lead , the bullet is lengthened inside the case to achieve the same weight as its predecessor . The U . S . Marines adopted the Mk318 in early 2010 due to delays with the M855A1 . This was a temporary measure until the M855A1 was available for them , which occurred in mid - 2010 when the Army began to receive the rounds . Both the Mk138 and M855A1 weigh the same and have similar performance , and both have better performance than the M855 against all targets . SOCOM spent less money developing the Mk318 and is marginally better than the M855A1 in some situations , but costs more per round . The Army spent more developing the M855A1 which performs as well or nearly as well as the Mk318 , but is cheaper per round and has the advantage of being lead - free . While SOCOM constantly looks for better equipment , the Army and Marines have far more troops to supply and buys more ammunition . 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 . 56 NATO ' s lack of stopping power along with lack of controllability seen in rifles firing 7 . 62 NATO in full auto . Some of alternative cartridges like the 6 . 8mm Remington SPC focused on superior short - range performance by sacrificing long - distance performance due to relatively short engagement distances typically observed in modern warfare . Others , like the 6 . 5mm Grendel , are attempts at engineering an all purpose cartridge that could replace both the 5 . 56 and 7 . 62 NATO rounds . The 300 AAC Blackout ( 7 . 62×35mm ) round was designed to have the power of the 7 . 62×39mm to use in an M4 platform using M4 - type magazines . None of these cartridges gained any significant traction beyond sport shooting communities . While both the 6 . 8 SPC and 6 . 5 Grendel perform better than the 5 . 56 NATO , they have their own drawbacks , including lower muzzle velocity and decreased magazine capacity . The 6 . 8 SPC ' s short and light bullet has poor long - range ballistics and a poor ballistic coefficient , and the 6 . 5 Grendel ' s case has poor ergonomics and a larger case diameter . By late 2004 , the 6 . 8 mm SPC was in limited use with U . S . Special Operators . However , it was not adopted for widespread use due to resistance from officials on changing calibers and the development of the 6 . 5 Grendel , which had better accuracy past 500 meters . In 2007 , both the U . S . SOCOM and the U . S . Marine Corps decided not to field weapons chambered in 6 . 8 mm due to logistical and cost issues . General Purpose Cartridge . The conflict in Afghanistan has caused a fundamental shift in the consideration of small - arms cartridge use . Previously , the normal maximum range for small - arms engagements was around 300 meters . In Afghanistan , Taliban fighters with PKM machine gun s and Dragunov SVD sniper rifles launched half their attacks between 300 and 900 meters . Over a quarter of engagements took place between 500 and 900 meters . British soldiers were initially only armed with 5 . 56 mm L85A2 rifles , L86A2 light support weapons , and L110A1 Para light machine guns . The L85 / 86 had an effective range of 300 meters and the L110A1 had an effective range of 200 – 300 meters . Over half of small - arms engagements took place beyond the effective range of standard British infantry weapons , and 70 percent were beyond the effective range of a short barreled M4 Carbine . 5 . 56 mm rounds also have poor suppressive effects against concealed positions at those ranges . The suppressive effect of a small - arms bullet is directly proportional to the loudness of the sonic bang it generates , which in turn that is directly proportional to its size . 5 . 56 mm rounds have half the suppressive radius of 7 . 62 mm rounds , which can be further decreased by wind drift at long - range . Complaints are compounded by laboratory tests that reveal that 85 percent of SS109 / M855 bullets do not yaw until at least of penetration , which would be most of the way through the body . Weapons like the 7 . 62 mm Mk 14 Enhanced Battle Rifle and Mk 48 machine gun became increasingly utilized by U . S . troops , as their need for effective fire beyond 500 meters was considered worth the weight increase . British troops turned to carrying the 7 . 62 mm L7A2 GPMG and L129A1 marksman rifle . The 7 . 62 NATO cartridge is larger , heavier , and produces three to four times as much recoil as the 5 . 56 . This makes it difficult to fire in rapid semi - automatic mode and almost uncontrollable on automatic . Users of the SCAR - H battle rifle claim that only the first round of an automatic burst would hit the target . Retaining both the 5 . 56 for close - range fighting and the 7 . 62 for long - range has its disadvantages , as only half the squad would be well - equipped for each situation . This also hurts logistics and the ability to interchange ammunition , as a squad would be armed with magazine - fed 5 . 56 and 7 . 62 rifles , and belt - fed 5 . 56 and 7 . 62 machine guns . There are several ways to address 5 . 56 mm deficiencies . One is to improve the load of the bullet . The U . S . has developed the Mk 262 , Mk318 , and M855A1 to give better performance from short barreled carbines , improve barrier penetration , and give more reliable terminal effectiveness . However , they have similar exterior ballistics to the M855 and do not meet the need for long - range coverage , as the rounds are limited by the small size and power of the cartridge . Furthermore , the open - tip designs and fragmentation ability of the rounds may not be accepted by other NATO countries due to their interpretation of Declaration III of the Hague Convention . Another way would be to return entirely to the 7 . 62×51mm cartridge . The principle 7 . 62 round is the M80 , which delivers terminal effectiveness through its size and power . It does not yaw rapidly on impact and although it has a longer range , it has poor long - range performance due to its relatively light bullet , which sheds velocity quickly . A low - recoil loading could be adopted for rifles , like the bullet used in the Japanese Howa Type 64 , while keeping full - power M80 rounds for machine guns and marksman rifles . The disadvantages with this are interchangeability in combat would be difficult because the rifles ' gas systems would need to be adjusted to use the different ammo types , the ammunition load for riflemen would be doubled , and the weight of machine gun ammo would be the same . The open - tipped Mk319 loading has good terminal effectiveness and barrier penetration , but greater weight and recoil still limits the usefulness of 7 . 62 mm rounds . Another option would be to keep the 7 . 62 for long - range use , and replace the 5 . 56 for short to medium - range use . The 6 . 8×43mm SPC has better terminal effectiveness and barrier penetration with little increased weight or recoil . The 6 . 8 mm round was kept short and relatively light to keep its overall length the same as the 5 . 56 , so longer - range performance is limited . A more radical alternative is to create a new cartridge that can replace both rounds . This concept has become known as a " General Purpose Cartridge ( GPC ) " , which could create one common round for a dismounted infantry section . With the size and performance class of the 6 . 8 mm for short - range , balancing the right caliber and bullet could theoretically match the long - range performance of the 7 . 62 with much lower weight and recoil . There have been several attempts to create and adopt a general purpose cartridge . The U . S . had previously sought to adopt a reduced power , general - purpose cartridge with the . 276 Pedersen in the early 1930s . It was intended to replace the . 30 - 06 Springfield but was rejected in 1932 because of large stocks of available . 30 - 06 ammunition . The U . S . Army also felt it offered insufficient long - range performance , as their mindset was for long - range semi - automatic fire . The British created a reduced - power round with the . 280 British for trials to select NATO ' s first standard rifle and machine gun cartridge . The bullet had less weight and recoil than the American 7 . 62×51mm and had more energy at long range , but was defeated by the U . S . insistence of their cartridge to be powerful enough to replace the . 30 - 06 in machine guns . In the 1970s , the U . S . created the 6 mm SAW round for use in a squad automatic weapon . Because of logistics concerns over adopting a third rifle caliber , development was abandoned when a better 5 . 56 mm round was promised , which arrived as the M855 . A comparison of M80 7 . 62 mm , M855 5 . 56 mm , 115 gr 6 . 8 mm SPC , and 123 gr 6 . 5 Grendel shows very different performances . 7 . 62 mm starts out very powerful at the muzzle , but drops significantly out to 1 , 000 meters . 6 . 8 mm is more powerful than 5 . 56 mm , but also decreases with range due to its short bullet . 6 . 5 mm starts with similar power to 6 . 8 mm , but has slightly greater energy than 7 . 62 mm at 1 , 000 meters . The 6 . 5 Grendel ' s low - drag , lead - cored bullet is longer and more aerodynamic , giving it a flatter trajectory and less wind drift despite its lower muzzle velocity . 6 . 8 SPC and 6 . 5 Grendel were designed to be the same length as the 5 . 56 to fit in a converted AR - 15 rifle , constraining certain bullet designs . A new GPC in a new family of weapons would not be constrained . While the 6 . 5 Grendel appears to fill the role of a GPC , it achieves the required performance through a long barrel and heavy lead - cored bullet . Out of a shorter carbine barrel , it would need an even heavier round for the same ballistics . It is constrained by its overall length if a lead - free round were made , as it would need to be longer to be the same weight as the lead - cored round , which would require filling up more case space and losing propellant ; even more would be lost using tracer bullets . If a light hybrid polymer / metal were to be used , the thicker case would further reduce propellant space . The desire for lead - free cartridges and lighter case designs mean that even though the 6 . 5 Grendel has better performance than most existing rounds , it does not have the case capacity to match 7 . 62 mm performance . The United States Army Armament Research , Development and Engineering Center ( ARDEC ) Small Caliber Munitions Technology Branch released the results of tests carried out to determine the optimum caliber for a military rifle in March 2011 . 5 . 56 mm and 7 . 62 mm rounds were tested against 6 mm , 6 . 35 mm , and 6 . 8 mm rounds , all loaded with lead - free copper and steel bullets . Criteria examined included : penetration ; terminal effectiveness ; accuracy ; initial , retained , and striking energy ; wind drift ; stowed kills ; and recoil . The study found that the 6 . 35 and 6 . 8 calibers comprehensively outperformed the others in their overall balance of characteristics . In 2012 , a study from the U . S . Army Marksmanship Unit ( AMU ) concluded that the optimum cartridge for a future infantry carbine should have a length and diameter greater than 5 . 56×45mm , be 6 . 5 mm caliber , have a muzzle energy of , and use low - drag bullets ( G7 BC of . 250 + ) to provide better long - range performance over the 7 . 62 mm with more retained velocity and energy , a flatter trajectory , and less wind drift . A Small Arms Intermediate Caliber Study done by the Canadian Department of Applied Military Science as part of their Small Arms Modernization and Replacement Project shows similar findings . They compared the ballistics of NATO 5 . 56 mm and 7 . 62 mm with commercial 6 . 5 mm and 6 . 8 mm rounds . Their conclusion was that 6 . 5 mm rounds with low - drag bullets had the best long - range performance and resistance to external factors . Two 6 . 5 mm cartridge designs , one lead - free weighing and one lead - cored weighing , have both been calculated to outperform M80 7 . 62 mm and M855 5 . 56 mm out to 1 , 000 meters , with all fired from 20 in barrels . At that distance , the 6 . 5 mm rounds have higher velocities , more energy , and more armor and barrier penetration . 7 . 62 drops 15 meters and 5 . 56 drops 17 meters below the line of sight , while both 6 . 5 rounds drop 12 . 5 meters . Against a 10 mph cross - wind at that range , 7 . 62 is blown sideways 4 . 25 m and 5 . 56 moves 5 . 4 m , while the 8 gram and 7 gram 6 . 5 rounds move 3 m and 3 . 3 m respectively . Time of flight for 7 . 62 is 2 . 1 seconds to travel 1 , 000 meters , while 5 . 56 takes 2 . 2 seconds , and both 6 . 5 rounds take 1 . 9 seconds . The 5 . 56 cartridge has the advantages of being the lightest and having the least recoil , but poor performance at long range makes it incapable of fulfilling the role of a GPC . A general purpose cartridge used by U . S . forces would likely be lead - free and have a steel penetrator like the M855A1 . European cartridges would need to comply with their interpretations of the Hague and Geneva Conventions , likely having a jacket that fully encloses the nose and sides of the bullet and doesn ' t expand or readily fragment on impact . Ball rounds would need to penetrate intermediate barriers and maintain its trajectory after passing through . Armor - piercing and tracer rounds also need to be developed . The GPC could be utilized through the Lightweight Small Arms Technologies effort to develop cased telescoped ammunition and caseless ammo , as well as weapons to fire them . Creating a lighter form of ammunition and new weapons would be a missed opportunity if current projectiles were used with their known deficiencies . 5 . 56 mm NATO versus . 223 Remington . The 5 . 56 mm NATO and . 223 Remington cartridges and chamberings are similar but not identical . While the cartridges are identical other than powder load , the chamber leade , i . e . the area where the rifling begins , is cut to a sharper angle on some . 223 commercial chambers . Because of this , a cartridge loaded to generate 5 . 56mm pressures in a 5 . 56mm chamber may develop pressures that exceed SAAMI limits when fired from a short - leade . 223 Remington chamber . Brass casing . The dimensional specifications of 5 . 56 NATO and . 223 commercial brass casings are identical . The cases tend to have similar case capacity when measured , with variations chiefly due to brand , not 5 . 56 vs . 223 designation . The result of this is that there is no such thing as " 5 . 56 brass " or " . 223 brass " , the differences in the cartridges lie in pressure ratings and in chamber leade length , not in the shape or thickness of the brass . In July 2012 , the Army solicited a request for vendors to supply alternative cartridge cases to reduce the weight of an M855A1 5 . 56 mm round by at least 10 percent , as well the for 7 . 62 NATO and . 50 BMG rounds . The cartridge cases must maintain all performance requirements when fully assembled , be able to be used by the Lake City Army Ammunition Plant , must be manufactured in quantities totaling approximately 45 million per year . Polymer - cased ammunition is expected as a likely lightweight case technology . A hybrid polymer / metal version of a conventional cartridge case would be thicker than regular cases and reduce the amount of space for the propellant . Pressure . C . I . P . defines the maximum service and proof test pressures of the . 223 Remington cartridge equal to the 5 . 56 mm NATO , at . This differs from the SAAMI maximum pressure specification for . 223 Remington of , due to CIP test protocols measuring pressure using a drilled case , rather than an intact case with a conformal piston , along with other differences . NATO uses CIP pressure test protocols for their small arms ammunition specifications . Because of these differences in methodology , the CIP pressure of is the same as a SAAMI pressure of , which is reflected in US Military specifications for 5 . 56 mm NATO , which call for a mean maximum pressure of 55 , 000 PSI ( when measured using a protocol similar to SAAMI ) . These pressures are generated and measured using a chamber cut to 5 . 56 NATO specifications , including the longer leade . Firing 5 . 56mm NATO from a chamber with a shorter . 223 Remington leade can generate pressures in excess of SAAMI maximums . Chamber . The 5 . 56 mm 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 test ed 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 . 56×45mm NATO and . 223 Remington equally well . The dimensions and leade of the . 223 Remington minimum C . I . P . chamber also differ from the 5 . 56 mm NATO chamber specification . Using commercial . 223 Remington cartridges in a 5 . 56 mm NATO chambered rifle should work reliably , but until recently , it was believed this was less accurate than when fired from a . 223 Remington chambered gun due to the longer leade . Although that may have been true in the early 1960s when the two rounds were developed , recent testing has shown that with today ' s ammunition , rifles chambered in 5 . 56mm can also fire . 223 ammunition every bit as accurately as rifles chambered in . 223 Remington , and the 5 . 56mm chamber has the additional advantage of being able to safely fire both calibers . Using 5 . 56 mm 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 . 56 mm NATO , such as many commercial 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 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 NATO ammunition . It should also be noted that the upper receiver ( to which the barrel with its chamber are attached ) and the lower receiver are entirely separate 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 the 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 ( not to be confused with stripped uppers where the barrel is not included ) to support both calibers in order to satisfy market demand and prevent any potential problems . Comparison of 5 . 56 mm NATO versus 7 . 62 mm NATO . Benefits of the 5 . 56 NATO claimed over the 7 . 62 NATO are equal lethality against soft targets , half the mass and volume , reduced recoil , noise , and muzzle flash , better penetration of metal plates , flatter trajectory and shorter time of flight out to 700 meters , weapons chambered for it are lighter , and higher hit probability . Hit probability refers to a soldier being able to concentrate on stance , weapon control , aiming , and trigger pull in spite of their weapon ' s recoil and noise , which has a noticeable difference between the two cartridges . While the 7 . 62 mm has twice the energy of the 5 . 56 mm , it is only required if the target is protected by armor . Both rounds will normally pass right through an enemy out to over 600 m . A 5 . 56 mm round fired from a barrel has a flatter trajectory than a 7 . 62 mm round from the same barrel length . A 7 . 62 mm round from a 20 in barrel has the same trajectory as a 5 . 56 mm round fired from the 14 . 5 in barrel of an M4 . A typical 7 . 62 NATO - chambered weapon has a barrel length of 20 in , and reaches half of its velocity after of travel down the bore . Shortening the barrel for close - quarters use results in decreased muzzle velocity and increased muzzle pressure . Military cartridge types . SS109 / M855 . In 1970 , NATO decided to standardize a second rifle caliber . Tests were conducted from 1977 to 1980 using U . S . XM177 5 . 56 mm , Belgian SS109 5 . 56 mm , British 4 . 85x49mm , and German 4 . 7x33mm caseless . No weapon could be agreed upon , as many were prototypes , but the SS109 was found to be the best round and standardized on October 28 , 1980 . The SS109 was developed in the 1970s for the FN FNC rifle and the FN Minimi machine gun . To increase the range of the Minimi , the round was created to penetrate 3 . 5 mm of steel at 600 meters . The SS109 had a steel tip and lead rear and was not required to penetrate body armor . Barrels required at least a 1 : 9 in rifle twist , but needed a 1 : 7 in rifle twist to fire tracer ammunition . The U . S . designated the SS109 cartridge the M855 and first used it in the M16A2 rifle . The 62 - grain round was heavier than the previous 55 - grain M193 . While the M855 had better armor penetrating ability , it is less likely to fragment after hitting a soft target . This lessens kinetic energy transfer to the target and reduces wounding capability . The M855 is yaw dependent , meaning it depends on the angle upon which it hits the target . If at a good angle , the round turns as it enters soft tissue , breaking apart and transferring its energy to what it hits . If impacting at a bad angle , it could pass through and fail to transfer its full energy . The SS109 was made to pierce steel helmets at long range from the Minimi , not improve terminal performance on soft tissue from rifles or carbines . In Iraq , troops that engaged insurgents at less than 150 yards found that M855 rounds did not provide enough stopping power . In addition to not causing lethal effects with two or more rounds , they did not effectively penetrate vehicle windshields , even with many rounds fired at extremely close range . In Afghanistan , troops found that M855 rounds also suffered at long ranges . Although 5 . 56 mm rifles have an effective range of 450 – 600 meters , the M855 bullet ' s performance falls off sharply beyond 300 meters . The ranges are even shorter for short - barreled carbines . Half of small - arms attacks were launched from 300 - 900 meter ranges . An M855 fired from an M4 Carbine has severely degraded performance beyond 150 meters . M855A1 . On June 24 , 2010 , the United States Army announced it began shipping its new 5 . 56 mm cartridge , the M855A1 Enhanced Performance Round , to active combat zones . During testing , the M855A1 performed better than 7 . 62×51mm NATO M80 ball ammunition against certain types of targets ( particularly hardened steel ) , blurring the performance differences that previously separated the two cartridges . The US Army Picatinny Arsenal stated that the new M855A1 offers improved hard target capability , more consistent performance at all distances , enhanced dependability , improved accuracy , reduced muzzle flash , and higher velocity compared to the M855 round . Further , the Army stated the new M855A1 ammunition is tailored for use in M4 carbine s , but should also give enhanced performance in M16 rifle s and M249 light machine gun s . The new 62 - grain ( 4 g ) projectile or bullet used in the M855A1 round has a copper core with a 19 - grain ( 1 . 2 g ) steel " stacked - cone " penetrating tip . The M855A1 cartridge is sometimes referred to as " green ammo " because it fires a lead free projectile . The M855A1 was put on hold in August 2009 due to the experimental bismuth - tin alloy core exhibiting undependable ballistics at high temperatures . The US Army has since replaced the bismuth - tin alloy core with one of solid copper eliminating the heat issue . The United States Marine Corps purchased 1 . 8 million rounds in 2010 , with plans to adopt the round to replace the interim MK318 SOST rounds used in Afghanistan when the M855A1 project was delayed . On a media day at Aberdeen Proving Ground on May 4 , 2011 , reports were given about the M855A1 ' s performance in the field since it was issued 11 months earlier . One primary advantage given by the round is its consistent performance against soft targets . While the older M855 was yaw - dependant , which means its effectiveness depends on its yaw angle when it hits a target , the M855A1 delivers the same effectiveness in a soft target no matter its yaw angle . The new SMP - 842 propellant in the round burns quicker in the shorter M4 carbine barrel , ensuring less muzzle flash and greater muzzle velocity . The M855A1 was able to penetrate of steel plate at 300 meters . The round even penetrated concrete masonry units , similar to cinder blocks , at 75 meters from an M16 and at 50 meters from an M4 , which the M855 could not do at those ranges . Its accuracy is maintained and sometimes increased , as it was able to shoot a 2 inch group at 600 meters . February 2011 was the first time the M855A1 was used more than the M855 , and approximately 30 million M855A1 rounds have been fielded from June 2010 to May 2011 . The M855A1 was put to the test at the 2012 National Rifle Association ' s National High - Power Rifle Championship at Camp Perry , Ohio in August 2012 . The shooter for the Army was Rob Harbison , a contractor supporting small caliber ammunition capability development at Fort Benning Georgia . This was a special event for the Project Manager for Maneuver Ammunition Systems and the Army ' s Maneuver Center of Excellence as it was an opportunity to showcase the capabilities of the Enhanced Performance Round . With an M16 loaded with M855A1 ammo , Harbison fired a perfect 200 points in the Coast Guard Trophy Match , which is 20 shots fired from the sitting position at 200 yards , finishing 17th out of 365 competitors . He also scored a perfect 100 on the final string of ten shots during the Air Force Cup Trophy Match , fired at 600 yards from the prone position , which is 10 shots in a row within the 12 - inch , 10 - point ring at 600 yards with combat ammunition . Harbison was happy with the performance of the EPR , with his scores showing that the Army ' s newest general purpose round is accurate enough to go toe - to - toe in the competition with the best ammo that can be bought or hand - loaded . Harbison even said , " I don ' t think I could have scored any higher if I was using match - grade competition ammunition . " From fielding in June 2010 to September 2012 , Alliant Techsystems delivered over 350 million M855A1 Enhanced Performance Rounds . Since its introduction , the M855A1 has been criticized for its propellant causing increased fouling of the gun barrel . Post - combat surveys have reported no isses with the EPR in combat . A series of tests found no significant difference in fouling between the old M855 and the M855A1 . However , manufacturers have reported " severe degradation " to barrels of their rifles using the M855A1 in tests . The Army attributes pressure and wear issues with the M855A1 to problems with the primer , which they claim to have addressed with a newly - designed primer . The round does increase the chamber pressure from 55 , 000 psi to 62 , 000 psi . From June 2010 to June 2013 , issuing of the M855A1 Enhanced Performance Round removed 1 , 994 metric tons of lead from the waste stream . of lead are eliminated from each M855A1 projectile . Mk 262 . The Mk 262 is a match quality round manufactured by Black Hills Ammunition made originally for the Special Purpose Rifle ( SPR ) . It uses a Sierra MatchKing bullet that is more effective at longer ranges than the standard issue M855 round . Two versions of the round have been procured to date . Initial production runs , designated Mark 262 Mod 0 , lacked a cannelure . Subsequent production , designated Mk 262 Mod 1 , added a cannelure to the bullet for effective crimping . According to US DoD sources , the Mk 262 round is capable of making kills at 700 meters . Ballistics tests found that the round caused " consistent initial yaw in soft tissue " between 3 - 4 in at ranges from 15 feet to 300 meters . Apparently it is superior to the standard M855 round when fired from an M4 or M16 rifle , increasing accuracy from 3 - 5 minute of angle to 2 minute of angle . It evidently possesses superior stopping power , and can allow for engagements to be extended to up to 700 meters when fired from an 18 inch barrel . It appears that this round can drastically improve the performance of any AR15 platform weapon chambered to . 223 / 5 . 56 mm . Superior accuracy , wounding capacity , stopping power and range power has made this the preferred round of many Special Forces operators , and highly desirable as a replacement for the older , Belgian - designed 5 . 56×45mm M855 NATO round . In one engagement , a two - man special forces team reported 75 kills with 77 rounds . The Mk 262 has a higher ballistic coefficient than the M855 of ( G7 ) . 190 , meaning it loses less velocity at long - range . Hard target penetration is slightly decreased . Mk318 Mod 0 . Following early engagements in Afghanistan and Iraq , U . S . Special Operations Forces reported that M855 ammunition used in M4A1 rifles was ineffective . In 2005 , the Pentagon issued a formal request to the ammunition industry for “ enhanced ” ammunition . The only business that responded was the Federal Cartridge Company , owned by Alliant Techsystems . Working with the Naval Surface Warfare Center Crane Division , the team created performance objectives for the new ammo : increased consistency from shot to shot regardless of temperature changes , accuracy out of an M4A1 better than 2 minute of angle ( 2 inches at 100 yards , 3 . 9 inches at 300 yards ) , increased stopping power after passing through “ intermediate barriers ” like walls and car windshields , increased performance and decreased muzzle flash out of shorter barrel FN SCAR rifles , and costs close to the M855 . The first prototypes were delivered to the government in August 2007 . Increased velocity and decreased muzzle flash were accomplished by the type of powder used . The design of the bullet was called the Open Tip Match Rear Penetrator ( OTMRP ) . The front of it is a hollow point backed up by a lead core , but the lead core only goes about halfway down the length of the bullet , while the rear half is solid brass . When the bullet hits a hard barrier , the front half of the bullet smooshes against the barrier , breaking it so the penetrating half of the bullet can go through and hit the target . With the lead section penetrating the target and the brass section following , it was referred to as a " barrier blind " bullet . Special Forces role in counter - terror operations allow them to follow certain law enforcement guidelines , so they could use hollow point rounds without violating the Hague Convention . Officially designated the Mk318 Mod 0 " Cartridge , Caliber 5 . 56mm Ball , Carbine , Barrier " , and called SOST ( Special Operations Science and Technology ) ammunition , the 62 - grain bullet fragments consistently , even out of a 10 . 5 in barrel . The lead portion fragments in the first few inches of soft tissue , then the solid copper rear penetrates 18 in of tissue ( shown though ballistic gelatin ) while tumbling . Out of a 14 in barrel , the Mk318 has a muzzle velocity of 2 , 925 fps . In February 2010 , the U . S . Marine Corps adopted the Mk318 for use by infantry . To be fielded by an entire branch of the military , the round is classified as having an " open - tip " bullet , similar to the M118LR 7 . 62 NATO round . The SOST bullet uses a “ reverse drawn ” forming process . The base of the bullet is made first , the lead core is placed on top of it , and then the jacketing is pulled up around the lead core from bottom to tip . Conventional , and cheaper , bullets are made with the method of the jacket drawn from the nose to an exposed lead base . The reverse drawn technique leaves an open tip as a byproduct of the manufacturing process , and is not specifically designed for expansion or to affect terminal ballistics . The Pentagon legally cleared the rounds for Marine use in late January . The Marines fielded the Mk318 gradually and in small numbers . Initial studies showed that insurgents hit by it suffered larger exit wounds , although information was limited . SOST rounds were used alongside M855 rounds in situations where the SOST would be more effective . In July 2010 , the Marines purchased 1 . 8 million M855A1 Enhanced Performance Rounds in , addition to millions of Mk318 rounds in service , as part of its effort to replace its M855 ammo . 5 . 6mm Gw Pat 90 . The 5 . 6mm Gw Pat 90 or GP 90 ( 5 . 6 mm Rifle Cartridge 90 ) , is the standard round used by the Swiss military in its rifle , the SIG SG 550 . The cartridge is also known as the Cart 5 . 6mm F to the French and Italian speaking Swiss militiamen . The Swiss refer to the round as the 5 . 6 mm Gw Pat 90 , although it is interchangeable with the 5 . 56×45mm NATO and . 223 Remington round . The Gw Pat 90 is optimized for use in 5 . 56 mm ( . 223 in ) caliber barrels with a 254 mm ( 1 : 10 in ) twist rate . The Gw Pat 90 was designed for the SIG SG 550 when it came into production in 1987 , replacing the SIG SG 510 . Previous experience of a change in standard rifle had proved that changing the distance of fire for the training ranges was more expensive than the design of a new ammunition ; this prompted the design of a cartridge nominally capable at 300 meters . The cartridge was also designed to reduce pollution by controlling lead emissions . The bullet was originally clad with a nickel alloy jacket , however , this was found to cause excessive barrel wear , so in 1998 the nickel jackets were replaced with tombac jackets . In addition , in 1999 a copper plug was added to the base of the bullet to address environmental concerns . The ammunition is currently ( 2009 ) produced by RUAG Ammotec , a subsidiary of the RUAG group . The ammunition is manufactured in three variations : the standard FMJ round , the tracer round , and a blank round . The FMJ cartridge has a Copper - Zinc alloy case and uses a double base propellant . The bullet is a 4 . 1 g ( 63 gr ) tombac jacketed FMJ projectile with a G1 ballistic coefficient of 0 . 331 ( ICAO ) / 0 . 337 ( Army Metro ) . The projectile contains approximately 95 % Pb , 2 % Sb , 3 % Cu , and was designed for terminal ballistic instability . The required accuracy for Gw Pat 90 ammunition out of factory test barrels is 63 mm ( 0 . 72 MOA ) for 10 rounds ( 100 % radius measurement method ) out to 300 m . The Gw Pat 90 cartridge dimensions are in accordance with the civilian C . I . P . standards for the . 223 Remington C . I . P . chambering . The Gw Pat 90 is used both in the Swiss military and in sport shooting . The very high level of individual training in the Swiss militia ( every single soldier bearing a weapon has to shoot for qualification once a year ; see Gun politics in Switzerland ) and the overall use of the Gw Pat 90 by the many Swiss citizens who shoot in competitions and for amusement has resulted in significant input on its usage . Over 1 billion cartridges have been produced as of 2005 .