Assume the muzzle velocities are close enough to count — almost certainly the case for the large differences.
Then run both through a reliable ballistics calculator like http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi
Start out with the 100 yard zero you observed for each bullet. Check the drop at 500 yards.
My guess is that the BCs aren’t that far different, since the three-inch difference at 100 yards translates by geometry to 15” at 500 yards
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