7.62×51mm NATO
The 7.62×51mm NATO (official NATO nomenclature 7.62 NATO) is a rimless, bottlenecked rifle cartridge. It is a standard for small arms among NATO countries. First developed in the 1950s, the cartridge had first been introduced in U.S. service for the M14 rifle and M60 machine gun. Many other firearms that use the 7.62×51mm NATO remain in service today, especially various designated marksman rifles/sniper rifles and medium machine guns/general-purpose machine guns (i.e. M24 Sniper Rifle and M240 Medium Machine Gun). The cartridge is used by military personnel, on mounted and crew-served weapons  that are mounted to vehicles, aircraft, and ships. Development. Work that would eventually develop the 7.62×51mm NATO started just after World War I when the large, powerful .30-06 Springfield cartridge proved difficult to adapt to semi-automatic rifles. A less powerful cartridge would allow a lighter firing mechanism. At the time the most promising design was the .276 Pedersen. When it was eventually demonstrated that the .30-06 Springfield was suitable for semi-automatic rifles, the .276 Pedersen was dropped. Thus when war appeared to be looming again, only a couple of decades later, the .30-06 Springfield  was the only round available, and the M1 Garand provided U.S. troops with greater firepower than their bolt action-armed opponents. The Garand performed so well that the U.S. saw little need to replace it during World War II, and the .30-06 Springfield served well beyond the Korean War and into the mid-1950s. The .30-06 Springfield was officially replaced by the 7.62 NATO M14 in 1957. During the 1940s and early 1950s, several experiments were carried out to improve the M1 Garand semi-automatic rifle. One of the most common complaints was the limited-capacity, eight-round en -bloc clip, and many experimental designs modified the weapon with a detachable box magazine. Springfield Armory's "T20" rifle was a fully automatic version. Though not adopted, experience with a fully-automatic Garand laid the groundwork for its replacement. The test program continued for several years, including both the original .30-06 Springfield round and experimental cartridges. T65 series experimental cartridges. During the 1940s, the .300 Savage became the basis for experiments on behalf of the U.S. Military that resulted in the development of the T65 series of experimental cartridges. The original experimental case design by the Frankford Arsenal was  designated the T65 and was similar to the .300 Savage case, but with less taper. The experimental cases were made from standard .30-06 Springfield cases which gave a little less capacity than standard .300 Savage cases because the Frankford Arsenal cases had slightly thicker case walls. The later T65 iterations were made from shortened .30-06 Springfield cases and were longer compared to the original T65 case as the .300 Savage has a shorter case length than the resulting 7.62 NATO. The resulting cartridges provided a ballistic performance roughly equal to the U.S. military .30-06 Springfield service cartridge . Over forty years of technical progress in the field of propellants allowed for similar service cartridge performance – firing a bullet at with muzzle energy – from a significantly shorter, smaller case with less case capacity. The eventual result of this competition was the T44 rifle. When the United States developed the T65 cartridge, the British military took a different route. They had spent considerable time and effort developing the intermediate-power .280 British (7 mm) cartridge with an eye towards controllable fully automatic fire. The U.S. held to its desire not to reduce the effectiveness of individual aimed shots. The American  philosophy was to use automatic fire for emergencies only and continue to use semi-automatic fire the majority of the time. After considerable debate, the Canadian Army announced they would be happy to use the .280 but only if the U.S. did as well. It was clear the U.S. was not going to use the .280 British. The British did start introducing the .280 British along with the bull-pup Rifle No. 9, but the process was stopped in the interests of harmonization across NATO. The T65E5 (7.62×51mm) was chosen as NATO's standard cartridge in 1954. Winchester saw  a market for a civilian model of the late T65 series designs and introduced it in 1952, two years prior to the NATO adoption of the T65E5 experimental cartridge iteration under the 7.62×51mm NATO designation in 1954. Winchester branded the cartridge and introduced it to the commercial hunting market as the .308 Winchester. The dimensions of .308 Winchester are almost the same as 7.62×51mm NATO. The chamber of the former has a marginally shorter headspace and thinner case walls than the latter due to changed specifications between 1952 and 1954. This allows 7.62×51mm NATO ammunition to feed  reliably in rifles chambered for .308 Winchester, but can cause .308 Winchester ammunition cases to rupture when fired in rifles chambered for 7.62×51mm NATO. Adoption in battle rifles. The T44 rifle was adopted as the M14 rifle in 1957. Around the same time Britain and Canada adopted the Belgian FN FAL (L1A1 SLR British) as the L1 followed by the West German army designated as the G1. The Germans soon transitioned to a modified version of the Spanish CETME rifle by Heckler & Koch that was adopted as the G3. With all of these firearms, it was clear that the  7.62×51mm NATO could not be fired controllably in fully automatic because of recoil. Both the M14 and FAL were later modified to limit fully automatic selection through semi-automatic versions or selector locks. Efforts were also made to improve control with bipods or heavier barrels. While this was going on, the U.S. "Project SALVO" concluded that a burst of four rounds into a circle would cause twice the number of casualties as a fully automatic burst by one of these rifles, regardless of the size of the round. They suggested using a much smaller, .22 caliber, cartridge with two  bullets per cartridge (a "duplex load"), while other researchers investigated the promising flechette rounds that were lighter but offered better penetration than even the .30-06. When the M14 arrived in Vietnam, it was found to have a few disadvantages. The rifle's overall length was not well suited for jungle warfare. Also, the weight of 7.62×51mm NATO cartridges limited the total amount of ammunition that could be carried in comparison with the 7.62×39mm cartridge of the Type 56 and AK-47 rifles, with which the Vietcong and North Vietnamese Army soldiers were equipped. In addition, the originally issued  wooden-stocked versions of the M14 were susceptible to warping from moisture in tropical environments, producing "wandering zeroes" and other accuracy problems, which caused the adoption of fiberglass stocks. Fighting between the big-round and small-round groups reached a peak in the early 1960s, when test after test showed the .223 Remington (M193 5.56×45mm) cartridge fired from the AR-15 allowed an eight-soldier unit to outgun an 11-soldier unit armed with M14s at ranges closer than 300 meters. U.S. troops were able to carry more than twice as much 5.56×45mm ammunition as 7.62×51mm NATO for  the same weight, which allowed them an advantage against a typical NVA unit armed with Type 56-1s. In 1964, the U.S. Army started replacing their M14s with M16s, incurring another series of complaints from the British. Regardless of the M14 having disadvantages in jungle warfare, 7.62×51mm NATO rifles stayed in military service around the world due to several factors. The 7.62×51mm NATO has proved much more effective than 5.56×45mm at long ranges, and has since found popularity as a sniping round. For instance, M14 variants such as the Mk 14 Enhanced Battle Rifle and M25 Sniper  Rifle were utilized in the United States military as designated marksman and sniper rifles. Shorter, easier-to-handle 7.62mm rifles like the Heckler & Koch G3 stayed in service due to their accuracy, range, cartridge effectiveness and reliability. In addition, continued if limited use in infantry rifles is a logistical convenience given the preference for 7.62×51mm NATO across NATO for general-purpose machine guns. Specialized use. Sniper and designated marksman rifles. Specialized loadings were created for 7.62×51mm NATO-chambered sniper rifles. They used heavier and more streamlined bullets that had a higher ballistic coefficient than standard ball rounds , meaning they shed velocity at longer ranges more gradually. Loss of velocity is important for accurate long-range shots because dropping from supersonic to transonic speeds disturbs the flight of the bullet and adversely affects accuracy. The standard M80 ball round weighs 147 gr and from a M14 rifle and M60 machine gun has a muzzle velocity faster than the M118LR 175 gr sniping round. However, the M80 drops to subsonic velocity around , while the initially slower M118LR is supersonic out to due to its low-drag bullet. The 7.62×51mm NATO round remains in use in designated marksman rifles  such as the Heckler & Koch HK417, SIG 716, FN SCAR, L129A1, and Colt Canada C20 DMR to take advantage of the effective range and accuracy potential compared with intermediate rifle rounds. Designated marksman rifles have to be effective, in terms of hit rates and terminal ballistics, at application ranges exceeding those of ordinary assault rifles and battle rifles, but do not require the extended-range performance of a dedicated sniper rifle. For this, depending on the military, sometimes specialized 7.62×51mm NATO ammunition is issued to designated marksman. General-purpose machine guns. The 7.62×51mm NATO round nevertheless met the  designers' demands for fully automatic reliability with a full-power round. It remained the main machine gun round for almost all NATO forces well into the 1990s, even being used in adapted versions of older .30-06 Springfield machine guns such as the Browning M1919A4 from the WWII era. The .303 British Bren gun was also subject to conversion to fire the 7.62×51mm NATO round, the converted weapon being reclassified as the L4 Light machine gun. West Germany rechambered many of its WW2-era MG42s to create the MG2, and the succeeding MG3 is essentially a slightly modified version  of the same weapon. These have been replaced to a considerable extent in the light machine gun role by 5.56×45mm weapons, such as the widespread use of the M249 SAW, but the 7.62×51mm NATO fully powered cartridge is still the standard chambering for the minigun machine gun and general-purpose machine guns such as the M60E4, FN MAG/M240, HK21, MG3, AA-52, Vektor SS-77, UKM-2000 and MG5 and flexible mountings such as helicopters, jeeps, and tanks. It is also commonly found in coaxial mount applications such as found in parallel with the main gun on tanks. The characteristics  of 7.62 mm bullet types were not only researched in the 20th century, but were also subject to 21st century ballistic studies. Post-2010 developments. The U.S. Army developed an improved version of the M80 ball 7.62mm round, designated the "M80A1". The M80A1 incorporates changes found in the M855A1 5.56 mm round. Like the M855A1, the M80A1 has better hard-target penetration, more consistent performance against soft targets, and significantly increased distances of these effects over the M80. The bullet is redesigned with a copper jacket and exposed hardened steel penetrator, eliminating of lead with production of each M80A1 projectile . The M80A1 began fielding in September 2014. The Army plans to replace both the M80A1 Enhanced Performance Round and M993 Armor Piercing round with the "XM1158 Advanced Armor Piercing Round" beginning in 2020. The U.S. Special Operations Command plans to begin fielding of the 6.5mm Creedmoor cartridge in early 2019 to replace the 7.62×51mm NATO round in semi-automatic sniper rifles. Tests determined that compared to the 7.62×51mm NATO (M118LR long-range 7.62×51mm NATO load), the 6.5mm Creedmoor doubles hit probability at , increases effective range by nearly half, reduces wind drift by a third and  has less recoil. The same rifles can use the new cartridge, as their similar dimensions allow the same magazines to be used and the weapon only requires a barrel change. Cartridge dimensions. The 7.62×51mm NATO has 3.38 ml (52.0 grains) cartridge case capacity. The exterior shape of the case was designed to promote reliable case feeding and extraction in bolt-action rifles and machine guns alike, under extreme conditions. 7.62×51mm NATO cartridge dimensions. All dimensions in millimeters (mm). Americans would define the shoulder angle at alpha/2 = 20 degrees. The common rifling twist rate for this cartridge is  305 mm (1 in 12 in), 4 grooves, Ø lands = 7.62 mm, Ø grooves = 7.82 mm, land width = 4.47 mm. The primer type can be Berdan or Boxer Large Rifle. U.S. Army Research Laboratory (ARL) research papers on the influence of Berdan and Boxer primer spit-hole diameter on 7.62-mm cartridge performance concluded the primary advantage of a Berdan primer is that they are less expensive than a Boxer primer due to their reduced complexity. The ARL found there is little variation in the pressure-time curves between the different spit-hole configurations. Doubling the area  of the spit-hole or incorporating a Berdan style spit-hole with the same total area as a standard M80 round showed minimal effects on the overall performance. The standard Boxer primed M80 showed the best results. All measured differences are within one standard deviation, and are not significant. According to the official NATO EPVAT NAAG-LG/3-SG/1 rulings the 7.62×51mm NATO can handle up to P piezo pressure. The proof round pressure requirement is piezo pressure recorded in a NATO design EPVAT barrel with a Kistler 6215 transducer, HPI GP6 transducer or by equipment to C.I.P. requirements. The  7.62×51mm NATO cartridge approaches the ballistic performance of the original U.S. military .30-06 Springfield M1906 service cartridge. Modern propellants allowed for similar performance from a smaller case with less case capacity, a case that requires less brass and yields a shorter cartridge. This shorter cartridge allows a slight reduction in the size and weight of firearms that chamber it, and better cycling in automatic and semi-automatic rifles. The .30-06 Springfield M1906 round weighed , and the 7.62×51mm NATO M80 round weighs . 7.62×51mm NATO vs. .308 Winchester. Although originating from an identical preceding series of experimental  cartridges, the commercial 1952 .308 Winchester and the military 1954 7.62×51mm NATO chamberings have evolved separately but remain similar enough that they can be loaded into rifles chambered for the other round, but the .308 Winchester cartridges are typically loaded to higher pressures than 7.62×51mm NATO service cartridges. Even though the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) does not consider it unsafe to fire the commercial .308 Winchester rounds in weapons chambered for the military 7.62×51mm NATO round, there is significant discussion about compatible chambers and muzzle pressures between the two cartridges based on powder loads , chamber dimensions and wall thicknesses in the web area of the military compared to commercial cartridge cases. As the chambers may differ accordingly the head space gauges used for the two chamberings differ. Department of Defense Identification Codes (DODIC). This four-character alphanumeric code is used by the US Armed Forces and NATO to identify the cartridge, the cartridge type, and the packing method (cartons, clips, link belt, or bulk) used.