A gun operating system for firearms, particularly for a shotgun, includes a bolt/slide movable between a closed position prior to firing and an open position after firing and a link operatively connected to the bolt/slide. An inertial recoil assembly housed in the gun stock is operatively connected to the link and includes a first mechanical an/or gas spring having a higher spring deformation rate in series with a second mechanical spring having a lower spring deformation rate. Upon firing of the gun, the bolt/slide is adapted to move to the open position and the link is adapted to initially deform the first spring in one direction and, subsequently, the first spring is adapted to return to its initial position and move the link and deform the second spring in the opposite direction.