A barrel assembly includes a housing for attachement to a conventional driving tool. A barrel member having a through bore is slidably attached at one end to the housing and carries a stress plate retaining pad at the other end. A pre-assembled enlongated headed fastener and stress plate combination is inserted into the barrel assembly whereby, during driving, the head of the fastener is slidably guided by the barrel bore while the shank portion of the fastener is rigidly held by the locked stress plate assuring a perpendicular driving alignment between the fastener and the workpiece, such as a roof deck plate. Biasing means normally forces the barrel in an extended position relative to the housing and also permits the barrel and stress plate retaining pad to retract during a plate tapping sequence of the fastener to prevent thread stripping of the stress plate.