Fayor optical Rifle Scope Bubble Level for 1in/30mm Tube Riflescope Anti-cant used for Shooting and Hunting For 1inch/30mm riflescope tube,Mounts quickly and easily to the riflescope tube Type: the ring fits perfectly onto a 30mm/1in scope with tightly screws. Makes it easy to maintain a perfect horizontal hold during extreme-range shooting Viscous fluid: No matter what circumstance, it always exists one bubble It really make a difference when you shoot in a long range or shoot with an air rifle which has a big drop trajectory.