Especially when fired out of a short barrel, as with the M4. Still, I think the 6.8 would be a better compromise (everything is a compromise when you're an engineer) between range, accurracy, effectiveness and loadout weight. The older 5.56 stuff did suffer, both in accurracy and effectiveness at longer ranges. And for the same reasons, too slow to "tumble" as designed. Those longer ranges were rarely encountered in the jungle, but more common in the mountains or dessert. In cities it can be either way depending on the scenario. First Gulf War was in the dessert but was also much more of a mechanized affair, as was the initial invasion in 2003.
As I understand it, there aren't many serviceable M-14s left in the warehouses. They were either destroyed by Billy Jeff (lest they fall in to the hands of the CMP and be converted into civilian legal semi-autos) or have been issued. Either as is, or upgraded. They try to have a so called designated marksman, with a 7.62x51 "battle rifle", along on most patrols. Or so I read anyway.
In the short term, simple changes in bullet design could radically increase the effectiveness of the current caliber and rifle issued to 90% of troops. Russian ammo, for example, is far more effective with the hollow space-penetrator-driver design. But our JAGs won't permit our men to use that type of ammo. Beehive rounds are fine, cluster bombs are fine, but decent ammo for the troops is verboten. Go figure.