The Modular Upper Receiver (MUR) is a rigid MilSpec flat-top upper receiver made to fit all AR15/M16/M4 rifles and carbines. The MUR was developed to give the customer a MilSpec 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, such as the bolt carrier group and barrel assembly.