To prevent the generation of an ark plasma at a gap with a rail by deforming the bimetal structure of a solid-state armature part by the heat caused by current and the friction with the rail and deforming it towards the rail.

A solid-state armature part 2 is arranged as a unit at the back part of a projectile part 1, current is conducted through the solid-state armature part 2 and by the electromagnetic force generated by the current and the current that flows rails 12, the solid-state armature part 2 is accelerated forward. By the heat caused by the current and the heat caused by the friction with the rails 12, the bimetal structures 39 of the solid-state armature part 2 are deformed and the deformation is set to be in the direction of contacting the rails 12 in proportion to the heat. With this, when a large heat capacity metal part 11 and 11a are worn and gaps are caused with the rails 12, the generation of the ark plasma is prevented.