Explosive powder charge operated setting tool

Explosive powder charge operated setting tool comprising an axially extending piston guide (1) having a trailing end and extending axially in a driving direction of the tool, an axially elongated driving piston (2) coaxially with and slidably mounted and guided within said piston guide and having a trailing end relative to the driving direction of the tool, said piston guide (1) is closed at the trailing end thereof by a base part (3), said base part has an axially extending channel open opposite to the driving direction to an explosive powder charge chamber (5), said driving piston (2) has a depression in the trailing end thereof facing said base part (3), wherein the improvement comprises that said depression (10) is annularly-shaped and extends concentrically about the axis of said driving piston (2) and is semicircularly-shaped in axial cross-section, said depression has a radially outer diameter corresponding approximately to twice the diameter of the semicircular cross-section at the trailing end of said driving piston, and said base part has a front end surface facing the trailing end of said drive piston with a recess (7) arranged centrally of the axis of said piston guide (1) and having a diameter greater than the radially outer diameter of said depression (10) in said driving piston (2).