A firearm sighting system includes a rear sight unit having a plurality of separate sight elements adaptable for \u201czeroing\u201d the same gun with different ammunition. The cap includes a main surface covering portion formed substantially at the center of the polygonal plate and a plurality of holding piece portions formed by bending corners of the polygonal plate from an outer peripheral end of the main surface covering portion to a rear surface side of the main surface covering portion. First and second mounting fixtures are attached to the photovoltaic solar panels which are of a type and geometry enabling the solar panels to be removably appended to the strut rails. The closure may be cast as a single piece of plastic material and may be cast in the open position.