I've always wondered about this, I would like some feedback on the connecting of the concepts here. Here goes-

At the end of World War II, the Army did a study of infantry engagements in the European Theater. It found that the side that fired more rounds was usually the winner of the engagement. After the war, proposals for automatic infantry rifles were requested from various gun manufacturers. Among them were the M-14 and AR-10. Of course as we know, the M-14 was selected. However, firing full sized cartridges, the M-14 was very unstable while being fired fully automatically. This is where Eugene Stoner steps in. He designs a high velocity, high rate of fire, (relatively) low recoil rifle. The design features are based on the study from WW2- the side who fires the most rounds wins. Soldiers can carry many more magazines of ammo in 5.56mm than they can in 7.62mm. On top of that, the 5.56mm round is of such high velocity, that it does as much damage (or more) as the 7.62mm, by fragmenting inside the body. So up to this point, we have a rifle based on sound principles: Light weight, high rate of fire, low recoil, lighter weight ammo, capable of doing as much damage as larger rounds.

Then comes Vietnam. Problems with the M-16 abound. McNamara and his whiz-kids start the downfall of the M16. Another post-war study shows that troops are using too much ammunition. The study also goes on to suggest that it only takes 3 rounds to kill an enemy soldier with the M-16 (for 100% certainty). With the end of Vietnam, the U.S. Military adopts the M-16A2. The automatic firing is taken away, replaced by single fire and three round burst. But there is a problem. The cam used in the M-16 for the three round burst causes the rifle to be less accurate than if simply firing three rounds from a fully automatic M-16. So essentially, we have a single fire rifle now, chambered in 5.56mm. Also on the scene now is the M-4 Carbine. While still fully automatic, it has a massive design flaw. Utilizing a shorter barrel, the muzzle velocity is lower. At ranges of 50 yards, the bullet begins failing to fragment. We see problems with this in Mogadishu, where enemy combatants must be shot numerous times to even render them incapable of firing. The bullets are simply making ice-pick style wounds.

So now we have the military armed with a semi-automatic small caliber rifle, and an automatic but far less lethal carbine. The M-16 has now strayed away from the principles of both studies- That the side who fires the most rounds wins, and that it requires three rounds to kill an enemy soldier with 100% certainty. The M-4 has also strayed away from the concept of a lightweight high velocity round that causes its damages by explosively fragmenting inside the human body.