Abstract: A device for accelerating a projectile to an extremely high velocity includes a launch tube having electrodes engageable with the projectile assembly for applying a large electrical voltage to it, the projectile assembly includes a pair of travelling electrodes fixed to the rear end of the projectile and engageable with the launch tube electrodes during the travel of the projectile assembly through the launch tube. The travelling electrodes define a spark gap which, under the high voltage applied from the launch tube electrodes, forms a high-temperature, high-pressure plasma arc travelling with the projectile and effective to increase its acceleration.