The RCO scope and ARCO scope in Arma 3 had incorrect bullet drop.
Based on real life, the (ACOG/Marks 4/RCO)scope was mainly form by a middle point(Arrow,Dots,or st similar), and a vertical line + 7 short horziontal line（3 of the line have a number on its right)
So the small horizontal line with a number on the rights shows the range for aiming.
EX: a target's at 400m, you will use the horizontal line that have a number 4 on the rights, and aim on the target.
Here is the problems, the bullet drop aren't fit, especially RCO sights.
Using the RCO sights, when the target is at 400m, and using the line "4" to aim and shot the target,the bullets drop higher and hit around 415-430m.
RCO sights: 300m-700m(incorrect)[Issue: the Bullet drop higher[10m-30m)
ARCO sights:600-900m（incorrect)[Issue: the Bullet drop lower[5-15m])
Free to test it and report on this thread.
Using both tracer and normal mags for precise data[because the lights that
6.5 x 39 tracer made disappear when the range reach 500m
Make sure you and your targets are at the same elevation
(Bullet drop are able to affected by the elevation!)