Are you using wind flags?
I have shot some of my wose groups in a "Dead Calm." In a dead calm you can have all kinds of things going on and without flags you will never see it. It dont take much to affect a bullet. To add did you pay attention to Mirrage? Changes in mirrage, can affect how you see the target, and will affect your groups. There may be more changes than you are aware of. Changing charge weights will affect barrel harmonics, and you may be hitting diferent nodes. Could also be a scope problem.
Bookmarks