Square Profile Gets Into The Tight Spots Ultra-fine, steel bristles in a unique, square shape get into tight corners to remove stubborn receiver fouling fast, without damaging the firearm. Originally designed for the M60 machine gun; works equally well to scrub out baked-on carbon deposits from inside AR-15 and shotgun receivers. Heavy-duty, looped and wound, steel shank provides extra strength and rigidity. Threaded brass coupling won t mar lightweight, aluminum receivers.