8" wide at 1,210 yards is 3/4 MOA that is fantastic. On a NON-muzzle brake
rifle my experience has been that vertical is from muzzle rise or an inconsistant pressure on forend. But with a 3/4 MOA wide group, it seems you are doing everything right. Barrel temp that I've experienced will affect both directions of your group. You're there just got to tweak something small. I put muzzle brakes
on all my big guns and shot better with the brake, and I Boran Nitride coat all my bullets and it keeps the barrel dramatically cooler and reduces friction and barrel wear. Remember if you coat the load you have now the chamber pressures will be lower and velocity also. Coated bullets will require more powder but will end up with a higher velocity. I used to send my bullets to Superior Shooting Systems to be coated, but now they offer a complete TBN kit for you to do it at home in your tumbler. It's great to save the shipping both ways and be able to coat and load when ever you want and not wait for them to be coated and shipped.