OPTICS for 7.62x 39 vs 51

It all depends on what you're trying to do with either one.
If the purpose for either is CQB, then a T-2 sized optic would be a realistic handy optic size for that purpose.
Are you trying to put both of them in DMR roles? Are you exclusively shooting 500yds?
We need more information.

Edit: Welcome to the Hide. Read and learn as much as you can. There's a lot of knowledgeable folk on here.
 
what is a realistic optic size and magnification usable for these two classic rounds?
huh? x51 has been used for long range shooting and sniper operations for decades so optics clearly have a wide bearth. x39 is considered a 200m and in caliber so pick your poison; rds or lpvo

where is that star wars gif with the fish admiral?
 
I love my SBR 7.62x39 AR, run a Steiner M536 prism sight on it with a yoter C. My big 308 Aero M5 has a S&B 5-20 Ultra Short with a HISS.
Which reticle did you choose? And how far have you reliably and consistently hit using it?
Been going back and forth trying to decide for my 11" Bren 2 in x39 that will be my comp gun for The Tactical Games. I believe the furthest is 500yds that they go, and not 100% if that includes their Sniper Challenge ones as well.
Thanks for the feedback.