Another factor I'm surprised no one has mentioned is letting your barrel cool down between groups. I had the same issue sighting in my .243 for 300 yards. After 10 rounds, the barrel would start to heat and it would start shooting low at 300 yards. After 30-40 rounds, it would be 2 - 3 inches low at 200 yards. It took a while to figure this out. With a cold barrel, I can hit a gallon of water at 300 yardsconsistently.

I'll shoot a 3-5 shot group, then let the barrel cool. I bring the 22lr to pass the time while the barrel cools. Never adjust your scope when shooting a hot barrel.

As mentioned in prior posts, correct breathing, trigger squeeze, and windcompensationare all key factors when shooting at 300+ yards.