Hard to improve on this - Geissele AR-15/M4 Super Reaction Rod

The reaction rod is good. The Super is better. When torquing a barrel nut or breaking loose a tight one the Super with the little dogs in the ejector port hold the receiver so the little pin on the barrel extension and the slot in the receiver don't bear the load. Remove the dogs and it's a great work station for muzzle device on and offs and other upper assembly tasks. Geissele makes good stuff.