What kind of war was it designed for?
Against the Soviet Bloc, aka a Regular Army, where they would care about a fellow wounded soldier. The terrorists don't - meeting Allah is fine with them.
There's been many complaints currently by the troops about the lousy killing power of the M-16A2 & its variants (there's been Threads on these articles). There's been articles posted that the US Army is now looking into the development of a new Main Battle Rifle with more power to replace the M-16. (check banglist)
In the meantime, the M-14 with its .308 would fit the bill, especially with a modification to the 3 round burst wanted by the ammo cost bean counters. The original M-14 was unwieldy on 'full auto'.