Cut and paste from full article here on the 5.56 vs 7.62 cartridge http://www.globalsecurity.org/military/library/report/1986/MVT.htm
Fire power is normally equated with maximum "steel" on target, not with
maximum steel in the general direction of the target. Full
automatic fire with the 5.56mm NATO just as wasteful and
Confirming this view is the fact that second generation assault
rifles, such as the U. S. M16A2 and Belgian FN FNC, are not
employing a 3-shot burst control in lieu of a full automatic
capability.24 With this burst control feature, a thirty round
magazine produces only ten bursts. Do we need thirty rounds to
successfully hit and incapacitate ten enemy targets? Even with
3-shot burst control and the lower impulse of the 5.56mm
ammunition, shot dispersion is still too large to be effective.
Perhaps a single well-aimed 147 grain 7.62mm bullet would have
more effect than three rounds of 5.56mm fired in the burst
control mode. As a result, the lower recoil and impulse of the
5.56mm ammuntion does not provide greater fire power since full
automatic fire from an individual assault rifle is largely
ineffective and only wastes ammunition.