A hard compact material, such as rock, is broken by driving a longish mass body of relatively incompressible fluid, such as water, against the material to be broken. The mass body is caused to impact the material at a momentum required for breaking the material. The required momentum is obtained by adding the momentum of at least two fluid mass bodies. The longish mass body can be aligned with a drill hole by means of the drill bit and rod. The energy generator or fluid gun and the gun barrel can be made in separable units.