The .30 military rifle goes back quite a long ways, exemplified in the .30-06 round which is a "30 caliber" introduced in 1906. It was the standard for US troops at least in WWII and for a time after.
In seeking a lighter weight ammo, so that troops could carry more of it, the 7.62 NATO round was introduced using the same "slug" as the '06 but with a more aggressive powder and hence a smaller case. The 7.62 NATO is dimensionally and ballistically identical to the (US)commercial .308 Winchester.
Observing statistics that I no longer have in hand, the US Army Ordnance Corps determined that in the "American Revolution" an enemy casualty (killed or wounded) was produced for each (IIRC) 17.6 rounds fired. In WWI it required (memory is vague) about 180(?) rounds fired for each enemy casualty. In WWII the number was up to close to 1,000 rounds per casualty. In Korea (the last war at the time I saw the report) it required slightly more than 300,000 rounds fired to hit one enemy person. (And the 1,000 lb "grasscutter" and "bunkerbuster" bombs were each counted as one round.)
With the obvious mandate to "shoot more rounds" the "duplex bullet" was created, and was used in both 30-06 and 7.62 NATO bullets. A short "front" bullet had its tail end cut off at a slant to make it "swerve off somewhere," and a "rear" bullet had normal conical shape. Since obviously American troops couldn't hit what they aimed at, a "secondary" bullet that tumbled deliberately and went in "some random direction" had about the same odds of hitting someone as one aimed deliberately at where an enemy might be.
The 5.56 NATO round was created primarily, once again, to reduce the weight of the ammo so that more rounds could be carried. The lighter round made possible slightly higher rates of fire in automatic weapons, but at the highest rates reported the bullets still come out of the barrel a few feet apart, and successive rounds are (shown by extensive testing) unaffected by the preceding one(s). If controllability on the part of the person firing the weapon is removed as a factor, the later rounds may actually be more accurate, and less likely to tumble than the first one(s).
In the "jungle warfare" of Korea and elsewhere, and in the increasingly common "insurgent combat," it is much less likely that a specific enemy person will be seen and can be targeted individually, although often the "enemy presence" can be known within an area. By shooting lots of rounds randomly in the general direction of where the enemy might be, occasionally some of them get hit. Having lots of rounds to shoot is facilitated by lighter ammunition, and modern powders permit sufficiently higher velocities so that the "ballistic energy" delivered to a target remains close to what was obtained with older and heavier ammunition, at least up to useful range.
Contrary to what might seem logical, military doctrine throughout the world has held for many years that an injured enemy is worth 5 killed, because (in "civilized armies") it takes about 4 - or more - others to care for one wounded, and thus takes more persons "out of combat" than the one burial detail clerk required to collect the tags and do the paper work on a dead one.
The vastly improved US medical services in later periods in Korea and all subsequent "conflicts" has the military purpose of getting the injured off the battlefield and out of the way of the remaining fighting troops. It has the peripheral effect of producing higher survivability of the injured, and certainly has very high "moral" (i.e. propaganda) value. Unfortunately, the reduction in "troops out of action" per injured has made the relative value of injured vs dead less of a factor to enemies, and has lead to much more lethal tactics and weapons whose purpose is to kill as many as possible rather than just to remove them from the field of combat via sublethal wounds.
Unsubstantiated(?) rumors claim that some insurgents have attempted to eliminate the discrepancy in "injury burden" by simply "executing on the spot" any of their own too badly injured to take care of themselves, to equalize the advantage the more "modern" military has with its rapid evacuation capabilities.