Hey JW- how goes it. If you dial down and it goes away--it's mirage. You will know. 78 degrees- It will still happen. It is just something you have to deal with. I am sure in your comp shooting you have seen others with a heat shield on their barrels. It helps with barrel heat mirage,but just the outside heat will still produce it. You have 2 options
1- dial down
2- shoot through it.
I do # 2 myself. I know you comp shoot,so being in those shoe's you know you are on a time limit. It is not something you will be able to escape from. Practice in it and you will be fine. The 600 yards you shoot the wind shifting and changing will be the hardest thing to learn.
PS-- Bart- Here is another guy that shoots a Savage Axis in comp and does very well for groups at 600 yards
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NwCP- Performance Isn't Optional