The reason is the ballistic perormance of that bullet is great at longer ranges.
I am sure the .308 develops more energy at any reasonable range although at some point that SS109 would probably overtake it.
The problem with those M4's (I think that is what they are called) is the short barrels which really lower the velocity and energy of the .223.
well crap, they pulled my thread(I shouldn't have posted the entire article)
your test made me remember the first time I shot my Glock 23. Was out plinking with another buddy(he had a copy of an Army .45). We were shooting at a piece of old culvert pipe at about 35 yards. You could hear a clear gap between the time he fired and the impact. It was so noticeable that we were laughing at about it. FWIW he was shooting some cheap LRN reloads.
I was shooting some flat-point FMJ re-loads that I had picked up at a gun show. For the longest time I thought I wasn't even hitting the pipe. Turns out his bullets were splattering(somewhat expected)against the pipe and mine were penetrating it-through both sides!(completely unexpected!) And their impact was happening so much faster that we never could hear it...
I've ofter wondered how all the other available .40 S&W bullets would perform.
I also think as long as the enemy "insurgents" aren't willing to obey the Geneva convention, and until a more potent caliber is available, I see no problem with issuing our soldiers hollow points.
There is a reason the FBI HRT, etc. transitioned to the M-4 from the MP-5....penetration. At short range, the 5.56 won't reliably penetrate sheetrock without a lot of fragmentation.
At the ranges that count, the 7.62 holds together and goes through things and into people, which is good because people tend to try to hide behind things.