A piston, normally contained within a cylinder, is projected by electrical detonation of an explosive charge which is contained within the cylinder between the piston and a sealed end of the cylinder through which a detonation contact extends to the charge. A split ring, retained in an annular peripheral groove in the piston, is normally contained under compression within the cylinder, but is resiliently expandible to a diameter larger than the internal diameter of the cylinder when released therefrom by projection of the piston under the expanding gases of the detonated charge, and thereby becomes operative, by abutting engagement with the end of the cylinder, to lock the piston in its projected position, and to hold the reactive load of whatever machine part has been actuated by the piston.