Make sure you're aiming with your scope directly over the bore of your air rifle with this 11mm Bubble Level from Sun Optics USA. The level mounts on an 11mm rail and shows whether there is any cant in your hold, which can affect your accuracy.
Specifications
Length: 1.9" / 4.8cm
Diameter: 0.5" / 1.1cm
Weight: 0.3oz / 8.5g