Correcting for Gravity (Bullet Drop)

          I have learned that one of the most common factors that affect a bullet while in flight is gravity. Gravity is the force that pulls every object down to earth at approximately 9.8 meters per second, or 32 feet per second. Gun manufacturers have realized that bullets start dropping the second that they leave the barrel of the rifle, but most people do not know that the gun makers have developed a way to make gravity less effective on a bullet after it is fired. To do this, they added a slight upward slope to the barrel. This causes the bullet to actually rise above the original line of trajectory of the projectile. It may seem like a bullet is traveling in a straight line, but it is actually traveling in a very oblong "parabola" or curved path. Therefore, after the bullet is fired it raises above the shooters line of sight, then drops below the line of sight. When you are making a zero, the point of impact is where the line of trajectory and shooters line of sight cross paths. The picture below will help explain.

              To help you guess exactly how much a bullet will drop at a certain distance, a number of things must be known. First is the muzzle velocity of the specific weapon system you are using. For my example I will be using a standard 30-.06 hunting rifle which has a muzzle velocity of ~2,800 FPS. Another thing is the shape of the bullet. I will be using a streamline (or pointy) bullet rather than a blunt bullet. This is because the blunt bullet will be in the air longer, thus allowing gravity to have a greater effect. At 100 yards, your zeroed-in rifle should not drop at all. (Assuming you sighted your rifle in correctly) Lets assume we are shooting at 500 yards, this is equal to 1,500 feet, therefore the bullet will be in the air for approximately half of a second. In this half of a second, gravity should pull the bullet downwards around 16 inches. If we go back to our mil-dots we find out that at 500 yards, one mil is approximately 18 inches. Therefore to compensate for this drop, we would have to aim our crosshairs ~1 mil dot above the target before firing.
          

http://www.angelfire.com/tx5/snipersustainment/External.html