I'm a ar fan but your wrong about stopping power the 5.56 is ment to maime not kill thus tieing up resources having to gather the wounded the 7.62 is a devastating round and is really good and ripping threw anything in its path like you said
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Thats a myth, 5.56 was
not designed to maim and tie up resources, that came as a consequence of hits at certain velocities and barrel length due to the bullets light weight and slower velocity. Also that would be against Geneva Conv rules. 5.56 was designed because 1) the majority of infantry engagements are within 100-200 yds and modern military theory favors assault style weapons(auto fire). We wanted a smaller round to enable more control over the weapon during rapid fire, a lighter weapon and to carry larger amounts of ammo. Most of modern history the favored battle round was 30-06 which required a heavy rifle and long barrel. Unless it was used with a bipod or fixed rest, a large round is virtually impossible to control during auto fire (BAR is a good example)
It is horribly expensive to adopt a new round and new arms so its a slow process. The .308 suffers from the same problem the 30-.06 does, hence its used as a specialty weapon (hence the short main battle status of the M14).
The next new round will be a 6.9mm, to eliminate the odd performance of 5.56