One thing I have noticed with military members is that most of the people complaining about lack of 5.56 terminal ballistics is that many of them don't have real experience shooting 5.56 at the enemy. Another reason that service members bad mouth the 5.56 is that many times they simply miss or hit a not vital location. They immediately blame the ammo.
For example: One of my guys had a Super Sniper scope on his M16A4. He made a shot at a guy who was about 400 meters away. The guy went down, then got back up and ran off. He was complaining about the 5.56 as not being effective enough. I asked him if he had looked at the person he shot up close. He told me that he didn't look at the person up close because the person had run off. I told him that there was no way of knowing if he even hit the person and if he did, the most effective round in the world won't do anything if you don't hit anything important(ie. you might have grazed him). He agreed with that.
A few of my guys that went over there with me expressed doubt in the 5.56 prior to getting there. After we'd been there awhile, everyone who had expressed doubt said that they were impressed with the 5.56 and had no problem carrying it.
Folks, firepower wins firefights. The person or group who can lay down a strong base of accurate fire is likely to win. If the troops were all carrying .308s(7.26) they would not have the firepower necessary to win. You go through ammo very quickly in firefights and carrying the amount of 7.62 ammo needed to win a lengthy firefight would be prohibitively heavy. Those of us who carried M16s carried at least 8 lbs of just rifle ammo. If you took the same amount of a heavier caliber that same 8 lbs would be well over 25 lbs. Doesn't seem like a lot until you consider the load that the troops are carrying. My personal combat load there was the following: Rifle, Helmet, body armor (w/SAPI plates), LBV w/GPS, 8 mags, 1 frag grenade, 1 smoke grenade, one pyro pop up, first aid kit, 100 oz Camelbak, and a few other small misc. items. The total weight came to about 65lbs. My SAW gunners and my M203 gunners carried a lot more....probably near 90 lbs. So adding another 15-20 lbs in ammo would be a big deal.
There are always going to be stories about that one person who took many rounds of 5.56 and continued to fight. Those stories are rare. Most of the people getting shot over there are going down without a problem.
Don't let the stories of the large number of rounds fired per person killed surprise you. Current military doctrine involves using suppressive fire and suppressive fire uses ammo at a massive rate. Suppressive fire saves the lives of our troops. The 5.56 is a great suppressive fire weapon. 7.62 is great as well in its role. But our troops need to be carrying 5.56.