A kit for modifying a bolt carrier group actuating apparatus of a firearm manufacturer-configured with a gas-driven bolt carrier comprises a gas expansion assembly, an operating rod driven bolt carrier and an operating rod. The gas expansion assembly is configured for receiving combustion gases from an as-manufactured OEM gas block of the firearm and for facilitating expansion of said gases for generating a bolt carrier driving force. The operating rod driven bolt carrier includes an operating rod engaging lug. The operating rod driven bolt carrier is configured for being operably engaged within an as-manufactured OEM receiver body of the firearm with the operating rod engaging lug located in a gas tube lug receiving portion of the as-manufactured OEM receiver body. The operating rod is engagable between the gas expansion assembly and the operating rod engaging lug.