Anyway, to get back to my original reply. Even if the .308 won't work directly, there are plenty of uppers made whose cartridge/magazine will fit into the lower of a standard AR-15. They will give you a broad selection of power, range and accuracy by simply swapping barrels & bolts by pulling out two pins. It also means you don't need to resort to a compromise, single caliber rifle that may do a lot of things adequately but nothing extremely well. This versatility is one of the reasons the AR platform has achieved the popularity it has.