A countermass for use in a weapon to balance the forces arising when a launch able unit is launched from a launcher tube of the weapon. The countermass includes a first container enclosing a liquid. A countermass weapon includes a launcher tube open or openable in both ends. The launcher tube houses a launch able unit, a countermass including a first container enclosing a liquid, a pressure chamber, and a propellant charge that upon firing and combustion of the charge in cooperation with the pressure chamber and the countermass accelerates the launch able unit forwards in the tube at the same time as the countermass accelerates backwards in the tube. The first container is made of a non-rigid material. A second container is provided to cover or almost cover the first container. The second container is designed with damping characteristics.