Higoo&reg; Optics Rifle Scope Bubble Level for 30mm Tube Riflescope Construction:This bubble level is made by aluminum alloy, which makes it properly solid to against all impact And using a matte finish, which reduce glare so that you position can be camouflaged at all times Type: the ring fits perfectly onto a 30mm scope with tightly screws. Design:It is easy to install and to mount anywhere on scope tube left or right side, the ring is so light to carry and cannot block you view of hunting 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