Abstract: A projectile (1) containing particles of interference material is fired pneumatically from a launching tube (2). Compressed air, which is supplied to the launching tube and acts on the rear face of the projectile during firing, is conducted both to a loading chamber (19) wherein the air pressure is maintained even after the projectile has left the firing tube, and also via the loading chamber to an expansion chamber (20) in which, after a specific time-lag corresponding with a certain launch height on the trajectory of the projectile, an over-pressure builds up which causes the expansion of the chamber to be initiated with simultaneous expulsion of the interference material from the projectile. The compressed air may also be conducted to the chamber (10) in the projectile wherein the interference material is accommodated, tightly packed, in order to enhance the dispersion thereof.