[oldenuff2no]

NO the 223 lower will not work for the 7.62 NATO round. It is long enough to chamber the 7.62X39 Russian round but that round is anemic compared to the 7.62NATO/308 win cartridge.
I do not know what this guy is talking about. The new rifle the military is talking about is a Remington Bolt action rifle built on a model 700 magnum length action. It has nothing to do with the AR platform.

[Still Thinking]

That’s what I thought. A normal AR-10 lower is different from that of an AR-15. I couldn’t imagine why they’d have bothered if you could use the 15 lowers for both calibers.

[equalitybeforethelaw]

Not aware of the AR configuration you speak of. Spent 3 years designing the Operational Test for the Special Operations Combat Assuat Rifle (SCAR). This rifle (2 variants: heavy and light) was truly designed around modular concepts and could easily be adapted to any caliber you could wish to choose. Its built by FN Herstal. Don’t know the status of the program anymore, but it was a fine combat rifle and a suitable replacement for the M16.