An arrangement for a weapon including a countermass for reducing the pressure around the weapon. The countermass is enclosed in a countermass container with an openable cover provided with break indications. The arrangement brings about a symmetrical opening operation with favorable recoil in a better way than in previous constructions. This is achieved by the introduction of a guiding folding support arranged adjacent to the openable cover on the inside of the cover relative to the countermass container.