The AR-style Bore Guide slides in place of the bolt carrier group assembly while cleaning. This centers the cleaning rod while preventing solvent from entering the upper receiver and trigger mechanism. The AR15/M16 style bore guide can be used for all of the AR15/M16/M4 family of rifles. Will work for both 5.56mm/223 Cal. variants. Pro-Shot's bore guides are manufactured using Delrin. Delrin is chemical resistant and will stand up to harsh chemicals, providing continual use.