Do you know which shot of the string ends up as the flyer? First shot, last shot or somewhere in the middle? Last shot could be a barrel heating/stock fitment issue. First shot could be a stock fitment pre barrel heating issue. Somewhere in the middle could be a you issue! Of course if these weren't hand loads then it could be an inconsistent neck pressure and/or powder charge and/or primer issue. Good luck figuring out. - Bill