A gun comprises a frame and a framework on which at least some components of a firing mechanism are mounted. The framework is inserted into a cross cavity of the frame and means for locking the framework within the frame are provided. The framework holds the whole firing mechanism except for a trigger and a connecting lever extending from the trigger. The framework comprises two hangers adapted to be laterally inserted into the cross cavity. The hangers are arranged at a mutual distance and the components of the firing mechanism are assembled between the two hangers being connected to each other by means of a bridge.