A vibration-dampening polymeric sleeve is provided for installation within the tubular receiver extension of an AR-10 derivative assault rifle before the action spring and buffer are installed therein. The presence of the sleeve dampens noise and vibration caused by the action spring of an AR-10 derivative assault rifle slapping against the cylindrical wall of the receiver extension. For one embodiment of the sleeve, a generally rectangular sheet of laminar polymeric plastic material equipped with tabs at the rear thereof, that is rolled into a tube. The tabs, which are inwardly bent, are held against the closed, rear end of the receiver extension by the action spring, whether the spring is fully compressed or only partially compressed. For an alternative embodiment, the sleeve, a solvated liquid polymer compound is sprayed into the cylindrical chamber of an axially-rotating receiver extension and allowed to dry.