We switched from 7.62 to 5.56 back in the early 60's( I think it was under Secretary of Defense Robert McNamara) . The thought behind that is of course cost and the psychological effect on the enemy. Since 5.56 tends to wound more than it kills all the wounded would cause a negative psychological effect on the enemy. Therefore lowering their moral. Basically it was Bean Counter BS.
That was fine when we fought cultures that cared about their own wounded.
I don't know, being dead sure does put a damper on one's moral.