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.
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. 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.
