Your scope is balanced, keeping your gun balanced - Vortex Bubble Levels for Rifle Scopes

Whether or not you have your scope balanced it doesn't do you any good if you're holding your gun at an angle. Your target may be leaning. If you angle your rifle to align with your target, you no longer have a balanced scope. If you shoot from a bipod that's swiveled, this device tells you - at a glance - if your scope is precisely above your point of aim.
At 50 yds, the effect is minimal. But I start at 100 yd. on paper targets, and at 400yds. the slightest tilt magnifies the variation 8x the deviation you got at 50 yds.
Some shooters may not need this as much as I do. I've found this attachment has improved my accuracy immeasurably. For consistently tight groups at 500 yds this is required equipment.