a trigger device 9 so as to project into the bore 1a through the adjacent closed end of that bore a barrel 11 is slidably mounted in the receiver bore 1a for movement between a loading position and a cocked position ready to fire and has a charge receiving cavity 13 in the end adjacent the closed end of the receiver bore when the barrel is in the loading position it is located forwardly relative to the receiver so that a charge can be introduced into the aforementioned cavity through a window provided in an upper surface of the receiver in the cocked position the barrel 11 is located at its rearmost position so that the charge 15 is able to be struck by the firing pin 7 upon actuation of the trigger device 9 spring means is generally provided to hold the barrel 11 at a relaxed position located between the loading and cocked positions and which is such that the charge 15 cannot be detonated by inadvertant actuation of the trigger device 9 a drive piston 19 is sidably mounted in a bore 11a of the barrel 11 and a combustion chamber 21 is provided between a head portion 19a of that piston 19 and the end wall of the barrel 11 containing the charge receiving cavity 13 the piston 19 is of reduced diameter forward of the head portion 19a and thereby provides a clearance space 20 for receiving combustion products combustion products reach that clearance space 20 by way of a retractor slot 23 formed through the lower wall of the barrel 11 and which communicates directly with the combustion chamber 21 when the piston head 19a has moved a short distance towards the front end of the tool from the.