As far as I know, the .308 is listed as milspec because of the fact that the rifle it is modeled after, the M24, is .308. M24's are also available in .300 win mag, but not available in. .223 or 5.56 and not used by the military, therefore no milspec on the box. Milspec only designates that it is .308 because all M24's used by the US military are .308. As for the other question, the rifle is chambered .223 because it was a specific later addition to the lineup a few years after the .308 was released, for civilian rifles only.