7.62mm caliber - Wikipedia

Also known as a .30 caliber cartridge (pronounced "thirty").

Post WW II the armed forces under NATO initially adopted the "battle rifle" concept - full size rifles capable of single shot or automatic fire. To go with this concept NATO retained a 'large' caliber bullet at 7.62mm (equivalent to .308 Winchester[?]) called the 7.62x51 NATO to distinguish it from non-NATO 7.62mm bullets. The Soviets adopted a reduced-power cartridge, the 7.62x39mm, for use in their machine guns and submachine guns.

The actual difficulties of using this caliber (weight of ammunition and weapon as well as the difficultly of controlling the weapon on full auto) and the analyses from actual combat (especially Vietnam) lead to the dropping of this calibre and the acceptance of the American 5.56mm caliber. The old caliber is still in use within NATO for sniper weapons and more widely in non-NATO weapons such as the Kalashnikov AK-47.

The 7.62mm calibre is very powerful, able to penetrate far more than six inches in 'soft' targets with around twice the lethal range of the 5.56mm. The 7.62 NATO round can actually cause less damage than the 5.56mm, as the bullet passes straight through the body rather than fragmenting, causing the energy to be translated into tissue disruption within the depth of a normal human.

7.62mm refers to the diameter of the lands in the barrel. This caliber, or size of bullet, is normally .308", although Soviet weapons commonly use a .310" bullet, as do older British and Japanese cartridges.