NATO
NATO's 7.62 × 51 mm rifle cartridge was introduced in the 1950´s as its standard infantry cartridge, used in the US M-14 rifle and the Belgian FAL rifle that most of NATO adopted under one name or another (the British called theirs the L1A1, for example). The "battle rifle" concept called for a weapon capable of firing a full-powered round (full-powered in comparison to the military rifle cartridges that were common in the first half of the 20th Century, such as the 7x57 mm Mauser, .303 British, 8x57 mm German Mauser, and .30/06) on full automatic. The cartridge adopted was developed by Winchester / Olin firearms, and the Pentagon gave Winchester permission to sell a commercial version of the cartridge as a hunting rifle cartridge called the .308 Winchester. The .308 Winchester cartridge is very popular with American sportsmen both for long-range target shooting and hunting big game up to the size of elk or moose.
It was observed that there was no way to make a controllable 7.62 × 51 mm assault rifle light enough to be practical, though there was and still is some question of whether full-auto fire is necessary, helpful, or even desirable in a general-issue infantry rifle. In the early 1960s a very vocal faction in the US military was of the opinion that it was, though given that this faction consisted mainly of highly opinionated Air Force generals like Curtis "Bomb the Reds back to the Stone Age" LeMay (LeMay was an intelligent and competent man, but he had no experience in ground warfare, just opinions about it), it's hard to understand why Secretary of Defense McNamara listened to them, but he did, perhaps because he didn't have any combat experience either, and just a few years after issuing M14 rifles to the Army and Marine Corps, he ordered the US military to switch to the M16 instead (though many of the M14s remain in storage in armories in the US, and even in 2004 many of them have been brought out of storage and issued to the men at the sharp end in Iraq and Afghanistan, and rumor has it that the Marine Corps now wants three men out of every thirteen-man rifle squad to carry M14s).
The argument for the .223 cartridge was that weapons such as the FN FAL were necessarily large, somewhat heavy, and (due to the power of the cartridge) difficult to control in full-auto fire, though the need for that feature is debatable. Note that most US Army and US Marine infantrymen today carry M16A2 rifles with the autosear, trip lever, and ratchet sear removed, making them semi-auto only. A secondary argument was that smaller calibers like the 5.56 x 45 mm / .223 Remington allowed American infantrymen to carry more ammunition due to the lighter weight of the individual cartridges, but general issue of full-auto weapons in the new small-caliber round meant that though American infantrymen in Vietnam with M16 rifles could carry two and a half times as much ammunition as they could for M14 rifles, they went through that ammunition three or four times as quickly, due to the poor training and poor discipline that were the norm for draftees in Vietnam.
In the early 1970s, NATO then adopted the medium-powered Remington .223 caliber as 5.56X45 NATO, retaining the larger cartridge for certain sniper rifles and medium machine guns; some NATO countries also use the 5.56X45 in belt-fed light machine guns as well.
eace: