The piston gun has a trigger, nozzle tube (26) containing a coaxial nozzle needle and a coupling device between trigger and needle. A fluid-tight conical seat is mounted between the end area of the needle and the end area of the tube when the trigger is inoperative downstream of a mixer reaction path between the inner wall of the tube and outer side of the needle. Upstream of the mixer reaction path and downstream of the cavity between needle and tube is a restriction area with its cross-section decreasing towards the conical seat. The mixer reactor path comprises two sections of which the first smaller cross-section is upstream of the seat adjoined by the larger cross-section part. <IMAGE>
