A means and method of controlling the muzzle velocity of a gun launched projectile to ensure that the muzzle velocities of several rounds of the same type tend towards the same value or that of an individual round approaches some nominal value. A sensor means measures a parameter related to the muzzle velocity of a projectile and a control means instructs an electrothermal energy unit to discharge a fixed amount of energy into the gun barrel after a certain time delay, the time delay being derived from the measured parameter and being such to ensure that discharge causes the projectile to achieve a controlled muzzle velocity.