I am a veteran and after I was medically retired I carried ARs for work both domestically as well as overseas. I spent a lot of time reading and studying why we are seeing the current failures of the AR platform. I also felt I have a pretty good idea why the problems started to arise when they did. The reduced effectiveness of the AR platform started when the Department of Defense chose to use shortened barrels with a 7 twist.
The main wounding mechanism of the 5.56, both 62g and the 55g, is fragmentation. And it is generally accepted that fragmentation occurs at or above 2,600 fps. But in order to fragment the bullet must first tumble but with the 7 twist barrel the bullets are now more stable resulting in less of a chance of the bullet tumbling. Couple that with reduced velocity of the 14.5" barrel and it is a perfect situation for failure.
The Department of Defense picked the worst of both worlds. They reduced barrel length which reduced the possibility of velocity induced fragmentation. Couple that with a faster twist reducing the chance for the bullet to tumble in the first place.
The original iteration of the AR15/M16 used a 20" barrel with 12 twist rifling and the 55g bullets they shot were barely stable with a SG of 1.42. This meant that when it hit anything the bullet would almost immediately tumble. The velocity was somewhere above 3,100 fps and with the added velocity from being fired from a 20" barrel they would reliably fragment at farther distances. This is why it was so effective in Vietnam despite the malfunctions induced by design and powder changes. Compare the wounds from a Vietnam era AR to wounds from a modern AR and the difference will be obvious. Not saying the current ARs are not lethal, just saying they are no where near as lethal, especially at distance, as the earlier versions.
Most of our active military carry M4s with a 7 twist barrel that is 14.5" long. The velocity of the 62g M855 out of a 14.5" barrel is ~2,900 fps. When the minimum fragmentation velocity is ~2,600 fps this means that the bullet will not reliably fragment past 100 yards because past 100 yards the velocity drops below the 2,600 fps threshold. This is why we are hearing of all the problems overseas because most engagements are well beyond that distance. And when you couple the reduced velocity with the added stability of a 7 twist barrel you end up with 22lr type wounds.
The Marines use a 20" barrel. The velocity of M855 out of a 20" barrel is ~3,100 fps. That extends the range for fragmentation to at least 150 yards. Not a lot but this is why I feel you hear more Army and less Marines complaining about performance of the rifles.
The reason why the 77 grain bullets increase performance is because they reduce the stability of the bullet, resulting in tumbling and possibly fragmentation, as well as carry the energy for a longer distance because of the added weight.
Another reason for failure is the thinness of the typical Afghan. Even under the perfect conditions most bullets will not begin to tumble inside the body of a Afghan male because of how thin they are.
If I were in charge I would do away with the 5.56 altogether. But if I had to keep the 5.56 the 7 twist barrels would be replaced with a 9 twist. 9 twist will generally stabilize bullets up to 77 grains, at least far enough to reliably engage targets at typicaly engagement ranges. I would also increase the barrel length for the typical soldier from 14.5" to at least 18". I would also increase the projectile weight from 62 to at least 69g. But if I did not have to keep the 5.56 I would go to the 6x35 KAC which is basically a 221 Fireball shooting a heavier, higher BC 6mm bullet. I am actually in the process of building a bolt gun and will likely do an AR in the 6x35.
Here is a VERY good read on the subject.
http://www.mlefiaa.org/files/ERPR/Terminal_Ballistic_Performance.pdf
