The RBS is an advanced dual aperture durable and low-profile auxiliary rear sight. It can be used with any front sight and was designed for more accurate aiming and faster target acquisition. Windage is easily and quickly adjusted without tools. - RBS is readily mounted on any 1913 Picatinny rail making it ideal for use on M4 / AR-15 / M-16 platforms and on the FAB Defense K.P.O.S. - Advantages: - 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 - Compatibility: All 1913 Picatinny rails