The Sniper Tool Design Co. Mil Spec Left Hand Angle Co-Sine Indicator (ACI) is a vault solid, water proof precision instrument; quiet and made from aircraft grade aluminum, which is anodized in a flat black color. The Co-Sine numbers are Laser Engraved onto the body in five degree increments. The lens is shatter proof, shock proof and resistant to chemicals such as acetone and cleaning solvents. Completely mechanical, there are no batteries to fail and no electronics to fail. This version of the ACI is Guaranteed water-tight and is manufactured as per the US Military Directives.