Abstract: Railgun structure having a pair of spaced-apart electrically conductive rails. A source of electrical energy is connected to the rails. An armature vessel is positioned between the rails and is movable along the rails. The armature vessel contains an electrically conductive plasma or gas through which electrical current flows when the rails are electrically energized. When electrical current flows through the plasma or gas, the pressure of the plasma or gas within the armature vessel becomes very high. At least one of the rails is provided with a series of vent apertures along the length thereof. The armature vessel has an opening therein which comes into communication with the vent apertures as the armature vessel travels along the rails. Thus, a portion of the plasma or gas is released through the vent apertures, and the pressure within the armature vessel is controlled. Therefore, the armature vessel travels at a very high velocity.