#6 Metal to metal fastening, used in light-gauge sheet metal applicaitons. recommended drive speed of 2,500 rpm. #8 Metal to metal fastening, use in light sheet meatl and light-guage steel framing, Very common in H.V.A.C. ductwork assembly. Recommended drive speed of 2,500 rpm. #10 Metal to meatl fastening, use in medium guage metal framing. commonly use by metal framing and H.V.A.C. contractors and fabricators. Can attach 12-20 gauge steel. Recommended drive speed of 2,500 rpm. #12 metal to metal fastening, used in sheet metal to attach to purlins and channels. Can attach 12-20 guage steel. Recommended drive speed of 2,000 rpm. #14 Metal to metal fstening, used for heavy-gauge metal attachment. Attaches 8-18 guage metal. Recomended drive speed of 1,750 rpm.