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 .