Geissele Automatics 05314 Reaction Block Black Steel AR15/M4/M16The 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 oclock 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