A cap assembly for an automatic cap feeder and fastener driver comprises a plurality of relatively thin, domed metal discs that can be pierced by fasteners discharged from a fastener driver such as a nail gun without bending the nail or jamming the driver, with the caps being connected together by a paper backed adhesive tape mounted on the undersides of the concave caps, with the caps having outer edges that are sufficiently sharp that they sever the tape from the strip of caps when the caps are driven into a substrate after a fastener has penetrated the caps. The fastener driver includes improved feeder teeth and an anti-backup pawl that extend into recesses in the track. A cover encloses an open edge of a cap storage basket.