Abstract: An apparatus for use in the generation of impulses, comprising a stepped piston assembly having a large piston and a small piston mechanically linked to one another and reciprocable between forward and rearward positions with the large piston being located in a large cylinder and having a large rearward face in communication with a reservoir for compressed fluid, and with the small piston having a small forward face for delivering the impulse; wherein the stepped piston assembly has an auxiliary piston located between the large piston and the small piston which auxiliary piston is mechanically linked to the large piston and which is reciprocable in an auxiliary cylinder, and wherein there is provided a valve through which fluid can flow from the volume enclosed by the large cylinder forward of the large piston into the volume enclosed by the auxiliary cylinder rearward of the auxiliary piston.