CMMG DISSENT Mk4 5.56x45mm NATO 6.5in Upper Receiver 1-7 Twist .750in 1/2 X 28 TPI Armor Black 55B9331-AB. 5 this DISSENT upper group is one of the shortest 5.56mm available to date. Packing all the power of 5.56 in a platform this size doesn't mean...