An upper receiver assembly for a firearm having an upper receiver with an integral barrel nut, a assembly and a barrel nut assembly with a barrel and lock nut is provided. The barrel is received by the upper receiver and is secured directly to the upper receiver using the lock nut. The upper receiver also includes an integral mounting member to which the assembly may be directly attached. The upper receiver assembly allows the user to attach both the barrel and assemblies directly to the upper receiver, independently of one another.