Doesn't DPMS make an AR-15 based .243?
It would be an AR-10. A .308 case will fit in a AR-15 magazine with about 1/4" to spare,the AR-15 just isn't long enough for a .308 parent case.
FWIW though, you can buy a .50 BMG upper for an AR-15, though the rifle would be a single shot.