In general terms, a "7.62mm" bullet is a "30 caliber" bullet. A .308 cartridge is sometimes called 7.62 x51 or 7.62 nato. (more?)
7.62 mm caliber
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 The M14 was superseded in U.S. service as the infantry adopted the 5.56×45mm NATO M16.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.
7.62 mm caliber is a nominal caliber used for a number of different cartridges. Historically, this class of cartridge was commonly known as .30 caliber, the Imperial unit equivalent, and was most commonly used for indicating a class of full power military main battle rifle (MBR) cartridges. The measurement equals 0.30 inches or 3 decimal lines, written .3" and read as Three-Line.
7.62 mm refers to the internal diameter of the barrel at the lands (the raised helical ridges in rifled gun barrels). The actual bullet caliber is normally .308 in (7.82 mm), although Soviet weapons commonly use a .311 in (7.91 mm) bullet, as do older British (.303 British) and Japanese cartridges.
North Atlantic Treaty Organization
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.
Coordinates: 50.8761556°N 4.4220111°E / 50°52′34.16″N 4°25′19.24″E
The North Atlantic Treaty Organization (NATO; // NAY-toh; French: Organisation du traité de l'Atlantique Nord (OTAN)), also called the (North) Atlantic Alliance, is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 4 April 1949. The organization constitutes a system of collective defence whereby its member states agree to mutual defense in response to an attack by any external party. NATO's headquarters are in Brussels, Belgium, one of the 28 member states across North America and Europe, the newest of which, Albania and Croatia, joined in April 2009. An additional 22 countries participate in NATO's "Partnership for Peace", with 15 other countries involved in institutionalized dialogue programs. The combined military spending of all NATO members constitutes over 70% of the world's defence spending.