The 7.65×51 ammunition cartridge was developed for the North Atlantic Treaty Organization (NATO), which was founded essentially as an anti-Soviet group to counterweight the Soviet Union’s control of the eastern Europe. NATO wanted to have a standardized rifle cartridge that could be used among all their forces.
Using the .308 Winchester cartridge as a base, the round was modified slightly. They wanted to eliminate feeding issues and to reduce overall cartridge pressure .
The 7.62×51 or 7.62 NATO (which is another common name for the caliber) was officially introduced into military service in conjunction with the M14 rifle. This rifle would go through multiple modifications throughout the years.
7.62×51 was first released in 1954, but American forces did not use the ammunition in significant fighting until the Vietnam war. In Vietnam, the military discovered the cartridge had a few disadvantages in jungle warfare.
Because of its size, troops could not carry as many rounds as the enemy, who carried the shorter 7.62x39mm. Also, the length of the M14 rifle made combat more difficult in tight jungle space.
Eventually, the M16 rifle and 5.56x45mm cartridge replaced the M14-7.62 NATO combination as the primary American weapon of that conflict, but the modified .308 Winchester remains an important part of United States and European military forces to this day.
Military forces from the U.S. and other NATO countries still use the 7.62 NATO, although it is primarily used for mounted-machine-gun and sniper-rifle purposes. In most cartridges, you will find the 7.62 NATO with bullets at roughly 145 to 175 grains.
Side note: 7.62x51mm is NOT the .308 Winchester, but it’s close. The .308 Winchester is generally loaded to higher pressures, but they are generally interchangeable in rifles chambered for one round or the other.