Angle Cosine Indicator
Also: ACI
A small device that reads the cosine of the shooting angle so a shooter can correct the drop on an uphill or downhill shot.
An angle cosine indicator, often shortened to ACI, is a gravity-driven gauge that mounts to the scope or rail and displays the cosine of the rifle’s inclination rather than the angle in degrees. Reading the cosine directly is convenient because the correction for incline shooting works by multiplying the slant range by that cosine to get a horizontal-equivalent distance. A shooter tips the rifle toward the target, reads a value such as 0.87, and multiplies it against the ranged distance.
The resulting shorter range is then used to look up the correct dope and dial the proper elevation, so the bullet lands on target despite the slope. Some ACIs are simple mechanical wheels while others are built into electronic levels or rangefinders that perform the math automatically. The device is most valued in mountain and field positions where steep angles are common and a quick, reliable cosine reading saves time.