Fast cutting aluminum oxide stone. Medium is perfect for fast production cleanup of the manufacturing burrs found in almost all new guns. A few quick strokes in the jig results in a much smoother trigger pull. Lubricate with stoning/honing oil. Fine is a good combination of speed and smoothness of cut to put a finer finish on parts for slicker, smoother trigger pulls. A favorite of many armorers. Lubricate with stoning/honing oil. Aluminum oxide.