Your hipfire sens does not equal your ADS sensitivity. The higher the zoom of the ADS the lower your sensitivity becomes. A way to fix that is to put your ADS sensitivity equal to the zoom level of the gun you're using. Unfortunately every gun has a different one. The ADS zoom for Vandal/Phantom - 1.25, Marshal - 3.5, Op - 2.5 and 5.0, Guardian - 1.5 etc. Obviously changing it mid game for every gun is not realistic. This problem might also be the reason why ADS on Vandal and Phantom is not popular as you're essentially forced to play on sens you're not used to.