PURPOSE: To allow changeover between semi-automatic mode and full--automatic mode and the next bullet is regulated when a bullet held by a packing is discharged, so that the bullets are discharged one by one.

CONSTITUTION: A cylindrical guide 22, that is rotated by manipulating a change lever 20, is mounted on a spring housing 10, and a pull-down piece 26 is connected to a bullet regulating piece 13 so that the spring housing 10 is connected to the guide 22 and rotated in the same direction as the guide 20 and the bullet regulating piece 13 is pulled down. A swing lever 29 is pivoted on a gun main body 1, facing the top face of the pull-down piece 26, and when the change lever 20 is located at semi-automatic mode position, it is rotated by pressing action of a trigger 5, causing the pull-down piece 26 to be pushed down.