Not to be confused with 7.62x39mm
7.62x51mm NATO is an American rifle cartridge used by various NATO countries in their militaries. The round is based off the .308 Winchester hunting cartridge, but 7.62 is loaded to lower pressures than .308.
The 7.62×51mm NATO (official NATO nomenclature 7.62 NATO) is a rifle cartridge developed in the 1950s as a standard for small arms among NATO countries. It should not be confused with the similarly named Russian 7.62×54mmR cartridge; a slightly more powerful round. It was introduced in U.S. service in the M14 rifle and M60 machine gun in the late 1950s. The M14 was superseded in U.S. service as the infantry adopted the 5.56×45mm NATO M16. However, the M14 and many other firearms that use the 7.62×51 round remain in service, especially in the case of sniper rifles, machine guns, and as the service weapon chosen by special operations forces. The cartridge is used both by infantry and on mounted and crew-served weapons mounted to vehicles, aircraft and ships. Although not identical, the 7.62×51mm NATO and the commercial .308 Winchester cartridges are similar enough that they can be loaded into rifles chambered for the other round, but .308 cartridges are typically loaded to higher pressures than 7.62x51mm cartridges for use in long-distance hunting of big-game animals. Even though the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) does not consider it unsafe to fire the commercial round in weapons chambered for the NATO round, there is significant discussion about compatible chamber and muzzle pressures between the two cartridges based on powder loads and wall thicknesses on the military vs. commercial rounds. While the debate goes both ways, the ATF recommends checking the stamping on the barrel; if you're unsure, consult the maker of the firearm.