Magpul has made an excellent reputation for themselves with their wide range of accessories and parts for real steel firearms. They are one of the first manufacturers that has embraced Airsoft as an acceptable form of training and simulation for real world simulation. Recently they have embarked on their own line of Airsoft accessories, such as stocks, magazines, etc.
The MagPul PTS MBUS Back-Up Sights are a great innovative flip sight system for the M4/M16. Both front and rear sight bolt firmly to the picatinny rail. Each sight is spring loaded in the down position with ambidextrous release buttons. this is a great back up sight for any gun with a continuous flat top rail.