Standard, Duct Fastener. Easy to install - place the liner over the sheet metal and drive the banger through the insulation. Upon impact, the jaws of the fastener penetrates the metal and locks securely within the thickness of the metal itself. (Always place a hardened steel block behind the sheet metal at the point of impact). Bangers have two prong jaws to fasten to the sheet metal. They are 10 gauge and zinc chromate plated.