There are several differences, but a few dominate. This is assuming 5.56x45mm NATO vs. 7.62x51mm NATO ammunition.For military use:(this assumes a group of soldiers, standard logistics, and automatic weapons, extended engagements or patrols)5.56mm is vastly lighter, both for the ammunition and the associated weapons system. You can carry roughly 3x as much 5.56mm as 7.62mm. Given that modern soldiers are carrying 50-100 pounds of non-weapons equipment, this is a big factor.7.62mm has greater range (800-1000m vs. 600m or so), although this is generally unneeded in modern combat. The kind of general-purpose assault rifle most troops are issued would be hard pressed at 800m even with 7.62mm ammunition. The latest 5.56mm long distance ammunition (77gr and heavier, e.g. the mk 262 mod 1) is more effective at longer ranges.5.56mm is much less powerful. For humans, this isn't always a major factor, especially in normal combat -- one round from either is likely to disable an enemy, if not immediately incapacitate. For suppression, 5.56mm automatic fire is basically as effective as 7.62mm, with the exception being performance through light cover, or against light vehicles. Again, the latest 5.56mm ammunition is much more effective against targets, even in close quarters.