A security device comprising: a barrier; and either (a) a plate, wherein the barrier and plate are releasably attachable to each other, and when the barrier and plate are attached the device comprises a slot for receiving a rail and the device may hang on a rail with the rail received in the slot, and the barrier protrudes in a horizontal direction, perpendicular from a plane extending downwardly from the rail, to provide an overhang, wherein at least a portion of the plate, but not the whole plate, comprises a bullet-resistant material, being at least a Type I bullet-resistant item, or (b) a panel, wherein, the security device may be transitioned repeatedly between a first configuration and a second configuration, in the first configuration the security device is unassembled and at least the barrier may be arranged in a storage configuration in which the barrier curves or bends through a first angle, and in the second configuration the security device is assembled in an arrangement for use, in which the security device may be assembled around a rail; and wherein, in the second configuration, at least a part of the panel is orientated to be substantially vertical, and the barrier is curved, bent or folded through a second angle, which is greater than the first angle, to protrude away from the panel in a horizontal direction, perpendicular from a plane extending vertically through the rail, to provide an overhang on an exterior side of the rail, wherein at least a portion of the panel comprises a bullet-resistant material, being at least a Type I bullet-resistant item, which is removably attached to the panel. a barrier; and either (a) a plate, wherein the barrier and plate are releasably attachable to each other, and when the barrier and plate are attached the device comprises a slot for receiving a rail and the device may hang on a rail with the rail received in the slot, and the barrier protrudes in a horizontal direction, perpendicular from a plane extending downwardly from the rail, to provide an overhang, wherein at least a portion of the plate, but not the whole plate, comprises a bullet-resistant material, being at least a Type I bullet-resistant item, or (b) a panel, wherein, the security device may be transitioned repeatedly between a first configuration and a second configuration, in the first configuration the security device is unassembled and at least the barrier may be arranged in a storage configuration in which the barrier curves or bends through a first angle, and in the second configuration the security device is assembled in an arrangement for use, in which the security device may be assembled around a rail; and wherein, in the second configuration, at least a part of the panel is orientated to be substantially vertical, and the barrier is curved, bent or folded through a second angle, which is greater than the first angle, to protrude away from the panel in a horizontal direction, perpendicular from a plane extending vertically through the rail, to provide an overhang on an exterior side of the rail, wherein at least a portion of the panel comprises a bullet-resistant material, being at least a Type I bullet-resistant item, which is removably attached to the panel.