The Geissele Reaction Rod is a tool that securely holds an upper receiver assembly for maintenance and assembly work. By gripping the Reaction Rod in a bench vice either horizontally or vertically, the upper receiver can be slid onto the rod and the rod's integral spines enter the barrel extension and secure the barrel extension from turning. This keeps all torque force away from your aluminum receiver, preventing damage to 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 receivers M1913 rail. Machined at Geissele Automatic's state of the art manufacturing facility in Norristown, Pennsylvania, cut from a solid bar of 4140 Chome Moly Steel and properly quenched/tempered and ground to an exact diameter that is smooth and straight so the rod will enter an upper receiver without wobble. Not compatible with HK MR762.Features: For AR-10 and SR-25 pattern upper receivers Protects receiver for installation of barrel nut machined from 4140 steel Made in AmericaSpecifications: Type: Action Block