For the most precise long-range shooting, use the bubble level to ensure that the rifle is perfectly vertical. This will eliminate potential error caused by shooting a canted rifle. Maximize your long-range shooting accuracy and precision by preventing a canted rifle with the Vortex Bubble Level for Riflescopes with 30mm Main Tubes. This simple addition to your scope will help you to ensure that the rifle is well aligned vertically, which will eliminate potential error caused by shooting a canted rifle. The mount is made from lightweight and durable aluminum with a black anodized finish and can be mounted quickly and easily on your scope.