In a firearm with a closing device of operative side ports of a receiver, comprising a bolt assembly (20) and a receiver (12) equipped on at least one side with an operational port (13), wherein the bolt assembly (20) comprises a bolt (21), a bolt-holder slide (22) carrying a cocking handle (24) and an inertial return element (23) of the bolt-slide (22), a closing device (30) comprises at least one cover (31) which can be moved along a longitudinal rail (32) from an advanced engagement position with the at least one operational side port (13) and a withdrawn disengaged position, wherein the inertial element (23) comprises a first spring (23A) and a second spring (23B) applied in series on the longitudinal rail (32) and in which supporting means (33) of the at least one cover (31) are assembled axially and slide along the longitudinal rail (32) between the first spring (23A) and the second spring (23B).