There is an axle (2) installed in the buttstock body. One end of the axle (2) is connected to the first end of the spring and the second end of the spring is fixed to the swivel platform. The swivel platform is being centered by means of one axle, the spring energy is transferred directly from the axle to the swivel platform. This makes possible to simplify the trigger mechanism of the swivel platform rotation and to increase the performance reliability.