[IronDuke99]

For anyone who does not know much about small arms, the small 5.56mm round mainly relies on high velocity to kill or wound, but that velocity falls off fairly rapidly with range and it also lacks penetrating power through obstacles like walls, trees or soft vehicles.

The US and the UK have both developed superior, more lethal, 5.56mm rounds in recent years, but the basic limitations of the 5.56mm round remain.

The 7.62mm round (and the NATO 7.62mm is a more powerful round than the short 7.62mm used in the old Russian AK47) has much better effects at longer ranges (ie, over about 300 yards) and much superior penetration through obstructions, through the greater mass of the projectile.

On the other hand 5.56mm is smaller and lighter, depending on the magazine used three 30 round mags of 5.56mm probably weigh roughly the same as two 20 round mags of 7.62mm. Infantry soldiers generally want to carry as much ammo as possible, but, since they are always loaded like mules, they also want to shave off all the weight they can.

5.56mm Performs well enough at under 300 yards and in the later stages of WWII that was the distance most infantry engagements occurred at, both in the villages and towns of Europe and the jungles of Asia and the pacific islands. In Iraq, and especially in Afghanistan, engagements were often at longer ranges, so 7.62mm has made something of a comeback in most Armies as a marksmen's weapon at section level.