An opening and closing device, associated with the rolling block and receiver of a hunting or sports gun, comprises a key that is hinged by a tube to the upper part of the rolling block, and a bolt that is actuated by a shaped end of the key and is slidably mounted on the rolling block in such a way that it can be moved from a retracted, open position in which the bolt is entirely inside the rolling block, and an advanced, closed position in which the bolt protrudes from the front of the rolling block with two arms designed to lock by friction two matching mortises made in the receiver; the arms extend from the ends of a shoulder of the bolt and the shaped end of the key has a first cam designed to exert a pushing action to the central portion of the shoulder in order to disengage the arms from the mortises.