ring 20 a barrel 22 slidable in the barrel housing and a captive piston 24 slidable in the barrel the piston embodies a relatively large diameter rear head 26 which is slidable in the barrel and a foward smaller diameter shank 28 which slides within an axial bore 30 in a barrel extension 32 mounted in the forward end of the barrel the piston is adapted to be driven by an explosive cartridge in the breech block not shown of the tool a return dog is mounted in the barrel housing in front of the head 26 of the piston the return dog serves to block forward movement of the piston when the barrel is thrown forwardly in the barrel housing when preparing the tool for firing when the barrel is thrown to its forward position the piston being blocked by the return dog becomes located in the rear portion of the barrel so that the nail 12 may be inserted into the barrel extension 32 the tool is then pushed against the work surface 14 pushing the barrel of the tool rearwardly so that the rearward end of the barrel engages the breech block in the handle housing conditioning the tool for firing when the operator pulls the trigger of the tool the firing pin in the tool is propelled against an explosive cartridge in the breech block causing the piston to be driven forwardly in the barrel thereby driving the nail 12 into the work surface whereupon the piston will assume the position illustrated in fig1 of the drawings the structure generally described above is typical.