The present invention relates to a method and a device by means of which it has been made possible, without gas discharge to the surrounding environment, to accelerate the speed of a powder-gas-driven projectile from zero to a speed which gives the projectile free-flying characteristics. According to the invention, this has been made possible by utilizing at least some of the expansion force from the propellant powder charge, which discharges gas when it is initiated, for, by a displaceable piston, driving a countermass out of the rear part of the projectile in the direction opposite to the desired flying direction of the projectile.