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Old 11-24-2011, 10:34 AM
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brian01tj brian01tj is offline
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Originally Posted by buggsb View Post
Really??? I'd love to see the math behind some of the comments here.

First, the effect of gravity on bullet drop is completely unrelated to the distance traveled.

Second, any "updraft" substantial enough to counter the gravitational drop would need to be huge.

The best instructors will tell you that if it can't be proved mathematically it's just opinion. There are plenty of myths and urban legends in this discipline which are assumed be fact just because they are repeated often enough.
Tailwind and Headwind will cause a POI chage because it has an effect on the velocity of the bullet. Since the bullet is seeing an increase or decrease in drag from the wind that means that the bullet is getting there faster/slower.

Here is a section from exterior ballistics:
" In Section 2.0 we discussed the two physical forces, gravity and air drag, which act on a bullet traveling through the air. The trajectory of a bullet is completely determined by these forces after it leaves the gun barrel. The gravitational force does not depend at all on wind conditions, but the air drag force does, and it has a very important influence on trajectory. Drag on a bullet is determined by the velocity of the bullet RELATIVE TO THE AIR THROUGH WHICH IT TRAVELS. When the air moves, the drag on the bullet is different from what it is when the air is still. It is just this drag force difference that causes the bullet trajectory in a wind to be different from what it is in still air.

This is quite easy to see when the bullet flies in a headwind or tailwind only (no crosswind). Suppose that you fire a bullet with a muzzle of 3000 fps and a tailwind of 10 mph. When the bullet leaves the muzzle its velocity is 3000 fps relative to the ground, since you are holding the rifle still relative to the ground. The wind at your back blows toward your target with a velocity of 14.67 fps (10 mph). Then, at the instant the bullet leaves the muzzle, its velocity RELATIVE TO THE MOVING AIR is 2985.33 fps. If there were no wind blowing, the bullet's velocity relative to the still air would be 3000 fps. Since the relative velocity is lower, the drag is a little lower when the bullet leaves the muzzle. As the bullet rides the tailwind, the drag is lower than it would be if the bullet flew in still air all along the trajectory. With less drag, the bullet reaches the target earlier (time of flight decreases), it has more remaining velocity when it gets there, and it suffers less drop (impacts a little high).

If you were firing into a 10 mph headwind instead, just the opposite situation would happen. The bullet velocity relative to the air would be 3014.67 fps at the muzzle. Since the relative velocity is higher than it would be in still air, drag is higher when the bullet leaves the muzzle. As the bullet bucks the headwind, the drag is higher than it would be in still air all along the trajectory. Consequently, the bullet reaches the target later (time of flight increases), it has a smaller remaining velocity when it gets there, and it drops more (impacts a little low). "

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