Abstract: In an explosive powder-driven setting gun in which a piston guide is axially displaceable within a casing and a driving piston is axially displaceable within the piston guide, a stop pin is provided for varying the volume of the combustion gases within the piston guide at the rear end of the driving piston. The stop pin extends between a locking piece in the rear end of the casing, through the rear end of the piston guide into contact with the rear end of the driving piston. An adjustable stop surface for the stop pin can be provided integrally with the locking piece, or as a separate member movably mounted in the locking piece. By adjusting the stop surface the spacing between the rear end of the driving piston and the rear end of the piston guide is determined for establishing the initial size of the combustion chamber for the explosive gases which propel the driving piston.