Magpul MBUS Rear SightMagpul MBUS Rear Sight Polymer Flip Up Backup Iron Sight The Magpul MBUS (Magpul Back Up Sight) Rear is a lightweight, polymer flip up backup iron sight designed for AR 15 and MSR platforms. Spring loaded for instant deployment, it folds flat when not in use to clear the way for your primary optic, then snaps up reliably when you need it. Adjustable for windage with a dual aperture design a large aperture for close range and low light use, and a