Excuse my n00bness. Jedi is from the AK camp.
I was under the impression that an AR-15 lower could take an upper that is chambered in .308 thus my AR-15 in .308 comment.
QUIT THINKING
The AR15 operates on the "micro" action size. The AR10 operates on the "short" action size. The limit, other than the operating length of the action in the upper, is the magazine well's overall length. A 308 size cartridge is just too long to fit into/feed through the AR15 length magazines.