some units in both the Marines and Army have, in a fit of common sense, instituted a designated marksman setup. As far as I can tell has been done both officially and unofficially, where one guy in the squad is chosen for his marksmanship abilities and given a rifle that can better make use of those abilities. Usually it is an M14, sometimes with a scope. I have also seen guys riding around Iraq with an M16 with a large hunting type scope, I would guess for the same purpose.
Traditionally, only the Navy seemed to have M14s, for shipboard use, but the limited supply of infantry M14s seems to be making a comeback lately.