I think the rationale was put in terms of weight and number of rounds carried. The lesson from Vietnam was that most rounds aren't even aimed -- it was several thousand rounds expended for each person hit. Also, the thinking at the time was that fighting was done at relatively close range, so the heavier bullet (and its longer range) was not necessary.
According to a book on snipers that I've read:
average bullets fired by US solders per kill
I think our military is much better trained than Viet Nam so there is less tendancy to "spray and pray" today