I'll post this here as a reference.
I'm putting a Vortex Strike Eagle 1-6X scope on my Sig556 Russian. The Vortex AR BDC is set up for the 5.56 obviously so the hashmarks won't be right for the rainbow 7.62x39. But if properly zeroed the AR BDC can be used out to 500yds very easily.
I did the calculations using Vortex ballistic calculator. The ammunition I chose for my calculations is Golden Tiger. Based on a BC of .300 and a velocity of 2375 and using a 50/150yd zero, here is what I get.
First column is yards, second column is the actual bullet drop, third column is the moa of the BDC hashmarks, fourth column is the difference.
100 . +0.9 .-0.0 . +0.9
200 . -1.5 . -2.4 . -0.9
300 . -5.6 . -5.6 . -0.0
400 .-10.9 .-9.5 . -1.4
500 .-17.5 -14.6 .-3.1