Can be used as primary or back-up sight Ergonomic angel-wing-shaped shroud helps center the image with any front sight and when used with a matching FAB Defense FBS Front Back-up Sight it creates a precise optical tunnel that ensures more accurate aiming and faster target acquisition Dual apertures are 3mm for longer eye relief mounting low light and faster target acquisition; and 1.5mm for shorter eye relief mounting and greater accuracy; each is instantly selectable to match conditions Low folded profile is smooth and inconspicuous and will not snag on gear webbing or brush Instant spring-locked deployment mechanism will not slip break or deform No tools needed for easy rapid windage adjustments with positive detent locking Made of extremely durable high-grade polymer and metal