IIRC, the deal with the 5.56 was to wound the enemy and cause a couple other soldiers to haul him out of the fight, thus reducing the fighting capabilities of the enemy, plus pound for pound, you couple pack 3X the ammo in the field carrying 5.56 v. .308 or 30-06. Both have their advantages/disadvantages.
Does anyone remember the Beltway sniper?
They used an AR in 5.56. They killed people far more often than injuring them. The round is deadly against people but using the long heavy for caliber bullets with a really fast spin results in a .223 sized hole. They will really penetrate but that is also the problem. They penetrate too easily.
The army ruined the M16 platform when they went to the fast twist rifling of 1 turn in 7 inches.Stoner’s original twist rate of 1 in 12 made the M193 bullet barely stable in flight.The bullet tumbled as soon as it hit,inflicting horrendous wounds.The M16A1 was probably the most deadly assault rifle ever built,at close range where you could use full auto effectively-your opponent was shreaded.I own an original class 3 M1A1,it’s effect on close range targets is amazing.