The Geissele Reaction Block is designed to aid in the assembly of lower receivers. Once the armorer has installed the buffer tube onto the lower; the buffer tube and lower receiver assembly can be slid into the Reaction Block at the 12, 3, 6 and 9 o’clock positions. This allows the armorer to rotate the lower in 90 degree increments in the block simply by loosening the provided nylon screws, removing the lower, and sliding it back in at the 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. All while keeping both hands of the armorer free to work, saving both time and frustration.