Abstract: This method serves to manufacture a coating gun, this gun comprising an injector for spraying a round jet along a spray axis and a cap arranged coaxially around the injector, this cap being provided to form an air knife around the jet. This method comprises a step consisting of sizing an axial excess dimension of the injector relative to the cap based on the diameter of the injector. The coating gun comprises an injector for spraying a round jet along a spray axis, a cap arranged coaxially around the injector, provided to form an air knife around the jet. The injector protrudes axially relative to the cap and the axial excess dimension of the injector relative to the cap can be adjusted between a minimum value and a maximum value that are determined based on the diameter of the injector.