Other Factors

            Throughout my research i discovered many variables that impact a bullet while in flight, some making huge differences, and others small. Wind, gravity, and drag all have a fairly significant effect on the projectile, but they are not the only ones shooters must keep in mind. Another important factor to consider is gyroscopic drift.

            All rifles have a thing called rifling, or small grooves in the barrel which put a spin on the projectile. This spin on the bullet stabilizes it in flight, much like how a quarterback puts spin on a football. This constant rotation causes the bullet to pull to one side. I discovered that 30-.06 bullet at 1000 yards will drift ~13 inches due to the rifling effect. So if we are shooting at 500 yards, we would have to use our mil-dot system to compensate for the 6.5 inch drift on the bullet. It may not seem like much but it could make a huge difference in combination with a strong wind and other determining factors.



            Another small factor is the coriolis effect. A french professor discovered the natural law which states that moving things such as projectiles and water move in relation to the natural spin of the earth. They move to the right in the northern hemisphere, and to the left in the southern hemisphere. What this means is that the natural spin of the earth could actually throw the bullet off of its original line of trajectory. I did some research and discovered that a projectile deviates about 6 inches at 1000 yards from the earths rotation in the northern hemisphere. However, maximum drift occurs when the bullet is fired towards the southeast and minimum drift when fired to the northwest. Generally a bullets curve will be about 1/10 of an inch at 125 yards in the central US, so there would not be a huge difference at distances around 500 yards.

http://www.appliedballisticsllc.com/index_files/SpinandCoriolisDrift.htm