A tool for driving nails into hard surfaces wherein the driving power is provided by cartridges of explosive powder. An extrudible stop member is provided in the barrel of the tool in front of the piston which is impacted by the piston when the tool is overdriven. The stop member extrudes through a passage to the front of the barrel if repeated overdriving of the piston occurs thereby providing a visual indication of such condition existing. In addition, a novel clamping arrangement is disclosed for retaining the return dog for the piston in the barrel housing.