Pardon if I missed it, but what length is the barrel? What gas length?
On a 20" you could stand going up to a #41 or even 42, maybe 43.
A 7.62x39 is going to need little bigger port than a 5.56. My references say a .078" port with carbine buffer is going to run a mid gas 16" 5.56. If that's the case, I'm guessing 3/32" is probably going to be a little too large.
