Anti-Cant Level
Also: Bubble Level
A small bubble level mounted on a riflescope or rail that warns a shooter when the rifle is tilted, preventing cant-induced misses at distance.
An anti-cant level is a bubble vial fixed to the scope tube, a ring, or the scope base, positioned so the shooter can glance at it while aiming. Its purpose is to reveal cant, the small left or right tilt of the rifle that is nearly impossible to feel but very real in its effect. By centering the bubble before firing, the shooter keeps the vertical reticle axis truly plumb with gravity.
Cant matters because a tilted rifle throws the dialed elevation off to the side, introducing both a vertical and a horizontal error that grows with distance. What looks like a windage miss at long range is often an unnoticed tilt at the shoulder. For shots where the bullet drops many feet, even a few degrees of cant can move the impact off target, which is why a level is standard equipment on serious long-range rigs.