It is strange that the rifle is shooting higher at 300 than at 100
Actually that is expected. With his excessive elevation, the rifle is essentially zeroed at about 600 yards, and at 300 the bullet is still going up, almost to the apogee of the trajectory.
You don't have tapered rings as well do you? 20MOA rings, plus 20MOA base could cause what you describe.
Assuming everything is put together properly, and properly torqued, you can try reversing your rings, see if that changes anything.