Just to clarify...If you have him build you a true 308 Win. then you are talking an AR-10 platform as opposed to an AR-15. The AR-10 is significantly bigger and IMO in most incarnations is more "battle rifle" than "assault rifle" to play fast-and-loose a bit, with those terms.
Nothing wrong with an AR-10, but it is bigger. The AR-15 is certainly perfectly capable of a .30 caliber cartridge or more as the 50 Beowulf proves that, but the cartridge itself needs to be shorter and will have a resultant decrease in velocity. This is where the whole idea for the 6.8 comes in.
