I have one brand new in the wrapper GG&G BUIS flip up rear sight with locking detent. I sold the gun before I could install it. Again, it has never left the wrapper. GG&G's item number is #1005.
The GG&G A2 Style BUIS (Back Up Iron Sight) provides the operator with an easy-to-install and easy-to-operate flip up rear sight, and now includes an important new locking detent. 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 A2 BUIS can be used as a stand-alone sight or in conjunction with one of our FIRE System rails. The A2 BUIS can be used as a co-witness iron sight with most of the red dot sighting systems. This sight features the standard A2-style double peep aperture and has full windage adjustment. It uses the 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. Initial zero is set by adjusting the weapons fronts sight post. When in the down position, it will clear most optics.
As an option you can request the XS same plane aperture be installed. Except for the Ashley same plane aperture, all sight components are standard M16A2 sight components. The A2 BUIS mount body and sight stem are manufactured from 6061 T6 aluminum and black hard-coat anodized per mil spec.
You can see pictures and read more at GG&G's web site at http://www.gggaz.com/products/a2stylebuis.php
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$80 plus free shipping.