The Magpul PTS MBUS2 Back-Up Sights are a great innovative flip sight system for the M4/M16. Both front and rear sight bolt firmly to the weaver rail. Each sight is spring loaded in the down position with ambidextrous release buttons. These are great back up sights for any gun with a continuous flat top rail. Magpul improved the design even more with their newest generation of the MBUS2. The lower Profile of the Gen.2 MBUS greatly improve optics compatibility due to their ability to fold to a profile of only approximately a half-inch. Magpul also improved on the design by making both the large and small aperture visible on the same plane of view.