I always wondered why the AR-10 was that much bigger and heavier than the AR-15.... Ruger has since proven that the 7.62 (308) can be chambered in a smaller rifle... basically the same size as the AR-15...
Given that fact, the only reason for going to the 5.56 would be the ability to carry more ammo.
I've always thought that the Russians had a better compromise in the 7.62X39... quite a bit more power than the 5.56, but still about the same in the "ammo carrying" department.
I looked for a durability test and found this article.
SFAR Review
