A modular rifle includes an upper receiver assembly and a barrel assembly. The upper receiver, typically fashioned from aluminum, can be strengthened by insertion of a hardened metal liner insert inside the aft portion of the upper receiver. The upper receiver liner insert is secured within the aft portion of the upper receiver assembly by engaging screw threads on the liner insert with receiving threads inside the aft portion of the upper receiver and insertion of anchor pins into the aft portion of the upper receiver. The anchor pins impinge the liner insert such that it is prevented from rotating within the upper receiver. The barrel assembly is attached to the liner insert by engaging screw threads on the liner insert with threads within a barrel nut connector sleeve.