The MUR is a rigid Mil-Spec flat-top upper made to fit all AR15/M16 M4 rifles and carbines. The MUR was developed to give the customer a Mil-Spec correct upper receiver with a thicker wall, making the MUR upper receiver a stronger platform. This more rigid upper receiver will increase accuracy and reliability, while extending the life of the interfacing components. The MUR-1A model comes equipped with a shell deflector and forward assist. The MUR-1S comes with a one piece machined shell deflector unit which was updated to this piece in June of 2015.