 Colt AR-15 rifles most often have a barrel threaded in 1⁄2"-28 threads to incorporate the use of a muzzle device such as a flash suppressor, sound suppressor or muzzle brake.The initial design, the "duckbill," had three tines or prongs and was prone to breakage and getting entangled in vegetation.The design was later changed to close the end to avoid this problem.