The PM137 AR-15 / M16 A2 rear sight can be used as a co-witness iron sight with most red dot and holographic sights, or as a stand-alone sight. Weighing in at just over two ounces, it features the standard military A2-style double peep aperture, which allows for a known point of impact. The sight has full windage adjustment; one click of the windage knob provides approximately 0.48 MOA (minute of angle) change when mounted on the M16 A3 flat top rifle and about 0.65 MOA on the M4 carbine. Initial zero is set by adjusting the weapon’s front sight post. It is low profile, so that when in the down position it will clear most optics. The sight locks in its upright position, a spring loaded release must be depressed to place the sight in the down position. The sight body and stem are machined from T6 aircraft grade aluminum, then type III hard anodized. Steel parts are CNC machined, then black phosphated per mil-spec. The sight is precision machined to fit AR-15 / M16 flat top A3 upper receiver rifles and carbines. The sight is mounted to the rail via a locking bar and secured with a cross slotted, knurled thumb screw.