Caliber is a description of the bullets width, not the length. 12.7mm is the equivalent of 50 cal so the M6D is a 50 cal. Also, the AR isn't necessarily more powerful, as muzzle velocity needs to be taken into account. As it stands the AR fires the exact same round as some modern assault weapons. It's basically just an AK-47. But with a shorter canon effective range, I believe.