First thing I'd do, is chrono the load in the brothers rifles to see what velocity he was getting ON THAT DAY. Next, I'd take a wind gauge, and see if there was measurable wind that could be playing with the bullet.
Then, I'd hold the scope steady, and move my eye around, and see what the parallax situation was. Then, I'd shoot s few more five shot groups with different rifle holds. It doesn't take much of a change in grip to change impact at 600 yards.
Every shot I made would be over the chrono. I have seen changes in temp change velocities enough to give more than a one inch change at 600 yards.
