I, personally, would have opted for a round similar to the .300 savage, which has better ballistics than the 7.62X39 round, is lighter than the .308(7.62X51)and was an existing round at the time.
Unfortunately, McNamara was the idiot in charge. Putting a brand new untested rifle into combat with the resulting jams and failures to fire should have landed that a**hat in jail. Yes, they got it working eventually but a lot of our boys were killed because their weapons wouldn't work before they did fix the problems with jamming and they still have a weak round that is even weaker in the short barreled M4.
Many years ago I was told I was an idiot because I thought the .250 Savage would make a great compromise round between the 5.56 and 7.62.
Somebody went through a lot of work to develop the 6.5mm Grendel in order to essentially replicate the ballistics of the .250 Savage :-)