This accessory provides an index point for blow-back pistol or DI caliber ARs and limits the movement of the lock collar on your barrel when not mated with your upper.
 
Note: The barrel stop is not compatible with two-piece pinned aluminum lock collars (2016 production). Lock collars produced prior to [October 2017] will need modified by drilling three holes (template coming soon) to allow access for an Allen wrench.  Your barrel profile must have at least 0.800” of flat surface with a diameter of 0.940-1.000” to install the barrel stop.