Doubt it’s spin drift as that wouldn’t be that much at 600. However could account for some but not all.
It’s most likely the wind. As it would not take much change in direction or speed to cause this effect. And there’s no way to accurately measure wind across the range before shooting and rely on that for the duration of the session.
Wind constantly changes. That’s why having experience reading it is so important. Wind flags help a lot if your range has them out.
I’d expect even your ballistic app to be off that much. I run StrelokPro and have been happy with it
However in case I’m wrong here’s a simple math equation to do before a shot that will put you right back in the game
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