I used a VX2 4-12x40 this year on 6.8 and worked great... 200 0 and down 9 at 300 so put cross hairs on top of back and the puts you in the middle. I have 4.5-18x44 on it now as it is just fun to shoot and easy to measure distance.
I bought an Armalite and then went back to a piston LWRC Six8...
https://www.theoutdoorstrader.com/threads/fs-armalite-ar10-spr-mod-2a-308-16-updated.1469359/
Great rifle but the Six8 suits my needs the best...