I coach a high school rifle team. They only shoot prone and with slings, gloves and peep sights. Stringing up and down is breathing issues, stringing right and left is trigger pull issues. Probably completely unrelated to your issue as you are using bags not the bones in your arm to support the rifle but I figured I'd throw it out there. Kind of automatic when I see that kind of vertical line. Their rifles are capable of putting the pellet through the same hole at 10 meters if it's in a vise.
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