KAC-Knight's Armament Picatinny Rail Mount Anti-Cant Device, Gen 2 (114294)
 
Picatinny rail mounted bubble level lets you know if the rifle is canted while shooting.  This is especially beneficial with long range shooting where a canted rifle can drastically change where shots are impacting.  
 
Unit sits approximately 0.4" above the top of the picatinny mounting surface. This replaces the KAC 30855 Anti-cant