I would suggest you buy a good quality variable that is at 24x at least, or better yet, a 36x fixed power unit.
Nightforce and Sightron seem to work well.
I have 2 - 36X Leupolds and one 36X Weaver on a 40X, a Suhl, and a M37. Both scopes are great for the long range game.
However, if you will be taking close shots as well, then a 6.5-20 glass with EFR may be just right.
It is absolutely amazing what one can do with a 22 rimfire and good optics at 200 meters on a quiet day.
The wind is definitely your enemy, though.
You will need the Signature rings with inserts, or a 20 or 30MOA base to reach out to 200 with most scopes.
See attached group from my 40X at 200. [the squares are ½"]
Don't even bother trying the long range with HV ammo. Find a Match ammo your 64 likes and use that.
Any other questions....just ask.

Dave.
