The Armorer's Action Block Kit wraps around the upper receiver and protects it from being crushed in the vise while you work on, or remove the barrel. The barrel vise jaw blocks are used to hold the barrel in the vise while working on the upper receiver. Only one would be used at any given time. Other necessities for this barrel removal job would be our Universal Armorers Wrench - for loosening and tightening the barrel nut and the U. S. Marine Corp. Tech. Manual.