The subject invention concerns a method and a device enabling said packed countermass material (11) to open up said countermass packagings (10) in relation to the firing of countermass weaponry. The basic concept for the subject invention is, thus, that one near the rear outlet end (12) of a countermass weapon (1) barrel fashion a number of blades (15) to the interior of the outlet that protrude inwardly into the barrel sufficiently so as to cut into said countermass container (10), which is pressed past said blades (15) upon firing said weapon (1).