7.62mm (.30 caliber) rimless bottleneck Rifle Round
7.62 is the (approximate) diameter of the rifle bore for the caliber cited (in millimeters). 51 is the length of the case (in millimeters).
Similar to the civilian .308 Winchester cartridge, the 7.62x51mm cartridge (also known as 7.62 NATO) is the standard NATO cartridge for .30 caliber machine guns and light sniper rifles in common use since the late 1950’s. Although the 7.62x51mm cartridge looks identical to the .308 Winchester, variations in the chamber dimensions and resulting pressures render them not interchangeable. It is generally safe to fire 7.62x51mm cartridges from a .308 Winchester chamber, but the reverse may not be true, depending on the loading. In keeping with “Better safe than sorry” .308 Winchester cartridges should not be fired from a rifle chambered in 7.62x51mm.