The main reason for moving away from the larger cartridges to the 5.56mm is that a soldier can carry roughly twice as many of the smaller cartridges for the same weight. This, along with the realization that most combat engagements took place within 200 yards (thus making the superior long-range performance of heavier bullets unnecessary), were the main reasons for adoption of the M-16.
The AKs are more reliable functionally, but are less accurate and have less impressive terminal ballistics, despite claims of "knockdown power".
This one?
I always thought the AK74 utilized the 223, or facsimile.