when you shoot rigles at distances of 1,000 yards and greater, there are lots of things that will make you bullet impact higher or lower. Here in south Mississippi in August, the heat can greatly effect where your bulloet hits in relation to your normal zero. Humidity, sunlight, wind and the rotation of the earth all effect teh flight of a miving, spining projectile as will teh uniformity of the projectile itself. You can buy all of the ballistic charts, ballistic computersand other doodads you want (I have most of them) but there is no better way to detrerming what you rifle/bullet combination will do than to shoot it in those conditions. That's why long range shooters are so anal about their data books. Some long range shooters are even starting to use video to record their data, shooting and the conditions.