The MAD BUIS (Back-Up Iron Sight) with Ranging Aperture was originally manufactured by GG&G under a contract with Picatinny Arsenal, and although it is ammunition and weapon specific, GG&G decided that it was time to offer it to a broader market. The GG&G MAD BUIS with Ranging Aperture has all of the features as the standard GG&G MAD BUIS such as a low profile, solid 4140 steel and billet 6061-T6 aluminum construction, solid locking detent with button release, and Mil-Spec finish. However, the rotating multiple aperture disc has five apertures preset to specified distances. In particular; 0=25 and 200yds, 3=300yds, 4=400yds, 5=500yds and 6=600yds. A small window facing the shooter provides a numerical indicator as to the aperture range. All apertures are the same .070-inch diameter. The apertures have been preset using the ballistic calculations for 55-grain through 62-grain 5.56mm ammunition used in the AR15/M4 16-inch carbine length weapon.