Upper Receiver is completely assembled and machined for M4 feedramps. The MUR was developed to fill the give the customer a Mil-Spec correct upper receiver, with a thicker wall thickness making the MUR upper receiver a more rigid platform. A more rigid upper receiver will increase accuracy, reliability and extend the life of interfacing components, such as the bolt carrier group and barrel assembly. The main benefit of the MUR is its dimensionally correct. Unfortunately, with the current supply effort, a number of AR suppliers are rushing through production, bypassing critical dimensions which will either make or break the reliability and accuracy of the weapon. With the existence of the MUR, you'll receive the best and highest quality upper receiver available on the market today. All MURs feature a MilStd-1913 scope mounting rail and come equipped with an Ejection-Port, Dust Cover Door.