An arrangement for a weapon including a countermass for reducing the pressure around the weapon. The countermass is enclosed in a countermass container that can be opened at both ends. The invention aims to bring about a symmetrical opening with favorable recoil in a better way than in previous constructions. This is achieved by the countermass including elements that can bind and retain the liquid symmetrically in a desired geometry over the cross section of the barrel and the cross section of any following expansion part.