The Geissele Reaction Block is designed to aid in the assembly of lower receivers. Once the armorer has installed the buffer tube, the lower receiver can then be slid into the Reaction Block at the 12, 3, 6 and 9 o'clock positions. This allows the armorer to rotate the lower receiver by 90° increments in the block simply by loosening the provided nylon screws, removing the lower receiver and sliding it back in at its desired orientation. The Reaction Block provides an ultra-stable platform for performing an array of lower receiver operations as well as making the installation of the buffer retainer nut quick and easy. This is all possible while keeping both hands of the armorer free to work, saving both time and frustration. Specifications for Geissele AR15/M4 Reaction Block: Manufacturer: Geissele Color: Black Gun Model: M4, AR-15 Condition: New Country of Origin: USA Package Contents: Geissele AR15/M4 Reaction Block What firearm models does the manufacturer say Geissele AR15/M4 Reaction Block is compatible with? The manufacturer says that Geissele AR15/M4 Reaction Block is compatible with AR-15, M4