ACOG scopes are able to determine the distance to the target using it's shoulder width. All you have to do is match the horizontal bar to the targets shoulders and see what bar is it. Bars are separated by increments of 100m and are marked by increment of 200, starting at 400m ([4]00-[6]00-[8]00) and the center of the scope is zeroed at 300m.
In game this is not correct. What happens is the distance marked on the bar is not the actual distance, but it's the suma of the distance and initial zeroing (screenshots below).
So for example if a target is 200m away (screenshot), the bar that matches targets shoulder width is the 5th one (500m) -> 500-300=200m -> actual distance.
I don't know if this kind of ACOG scope exists in real life, where you have to do quick math, but I think marking the actual distance on the scope bars would be more player-friendly.
TL;DR: RCO/ARCO scopes' distance finder doesn't work properly. {F18670} {F18671}