Actually the 6mm/.243 makes more sense for a new standard issue rifle than the .308 or larger caliber. Remember that it would be assigned/issued to all across services, different jobs from infantry to clerks and to men ans women. A 6mm/243. would be a big step-up from the .223 and would be a pretty good choice. Having more specialized rifles/firearms/weapon systems for more specialized job/MOS would also make sense.