The GG&G MAD (Multiple Aperture Device) BUIS (Back Up Iron Sight) was originally developed to fulfill a request from Naval Surface Warfare for a back up iron sight that provided both a large and small aperture on the same plane and would thus be zeroed to the same point of impact. The MAD has four apertures, two of each size (so that rotating the aperture in either direction will bring the next size into view). The small aperture diameter is 0.073-inch and the large aperture is 0.199-inch in diameter. It uses the standard Colt-style windage knob and windage screw. One click of the windage knob provides approximately 0.48 MOA change when mounted on the M16A2E4 rifle and about 0.65 MOA on the M4 carbine. The MAD mount body is manufactured from 6061 T6 aluminum and black hard-coat anodized per mil spec. The stem and aperture disc are made from 4140 steel, black manganese phosphated per mil spec. The MAD overhangs the rear of the receiver by 0.200-inch. This provides a lip so that the sight can be easily deployed even with a gloved hand. The unit locks in the up and down position with a new positive mechanical detent system. Once placed in the up position, the sight cannot be folded down without depressing the spring assisted detent release button located on the left side of the sight base. This totally eliminates the possibility of accidentally lowering the sight. The MAD is the lowest profile BUIS with a locking detent, and this item includes the Trijicon Tritium night sight insert option.