6 Metal to metal fastening, used in light-gauge sheet metal applications. recommended drive speed of 2,500 rpm. #8 Metal to metal fastening, use in light sheet metal and light-gauge steel framing, Very common in H.V.A.C. duct-work assembly. Recommended drive speed of 2,500 rpm. #10 Metal to metal fastening, use in medium guage metal framing. commonly use by metal framing and H.V.A.C. contractors and fabricators.