Angle cosine indicators are an invaluable tool for the long range shooter, and to ensure that your A.C.I. is calibrated correctly you must ensure that the bore of your rifle is level. Introducing the True Bore level from Flatline Ops. The True Bore attaches to the crown of the barrel with a high strength magnet and from there it's as simple as adjusting your rifle to center the bubble between the lines. This precision to allows you to level your bore with speed and ease to ensure that your A.C.I. is perfectly calibrated to help tighten those groups down range.