My dad never felt undergunned with a M16 in Iraq. Considering the fact that much of the fighting is in an urban environment, the M16 and its varients are a much more useful and handy weapon than the bulky M1.
American tactics since WWII have involved the use of heavy firepower. In fact, the Germans used to become irate because the Americans didn't usually engage them as infantry; the GIs blew the crap out of them with planes, artillery, mortars, rocket launchers, and granades if they could first, according to many firsthand accounts. Also, I believe it was Frank Salter who said that infantry was used more as a mop-up tool.
Obviously there are many, numerous exceptions and circumstances where the heavy firepower doctrines were not or could not be used. That said, if you can't kill something with a .223, chances are you couldn't hit it with a .308 or need a bigger weapon anyway.
And lest we think that a .30/06 means one shot, one kill, remember that post-war Defense Department estimates said that approximately 20,000 shots were fired for every dead enemy soldier.
Eddie