The AR15/M4 Reaction Rod makes the removal and installation of barrels, flash hiders, gas blocks and hand guards much easier and simpler. The Reaction Rod is designed to be gripped in a bench vise so that the rod is either horizontal or vertical. The upper receiver is then slid onto the rod and the rod’s integral splines enter the barrel extension and secure the barrel extension from turning. This allows all the torque from barrel nut wrenches to go directly into the barrel extension. In contrast, receiver vise blocks transmit the turning force into the aluminum receiver, a good part of which passes through the small, easily distorted receiver index pin. With the Geissele Reaction Rod, marring of an upper receiver’s finish by gripping and twisting inside vise blocks is eliminated and so is the need to remove sights and mounts from the receiver’s M1913 rail.