Abstract: An inertial impact switch is provided having a housing which comprises two onductive sections electrically insulated from each other and a movable contact member engaging one section and held out of contact with the second section by means of a concentric insulating member. Interior radial "fingers" of the insulator bend in response to the spin forces of the projectile containing the switch. To provide for this sensitivity, the fingers of the insulator may have varying stiffness to provide deflection under differing spin conditions. The body of the switch may also be provided with a single grove bellows that is compressible under applied stress prior to use, moving the first section of the housing closer to the second section of the housing, thus reducing the gap between terminals.