Ross3914 hit a lot of good points. Although the AR started life in .308, McNamara dropped it down to .223 to afford more ammo in a given space. Had the round stayed .308 and .223 introduced later, I suspect the situation would be reversed. As any vet who was around when the AR15 was introduced to the troops, and you will find it had a lot of teething problems that left them very undesirable, including feeding problems, which led to the forward assist. One of the problems turned out to be the ball powder that was being used, which caused debris that made chambering difficult to impossible.
So many S&W's, so few funds!