Originally Posted by Fitch
Wind drift is directly proportional to lag time. Lag time for a given bullet is defined as the difference between the flight time in a vacuum and the flight time in air to reach the distance of interest.
Wd = Ws * Tlag
Wd = wind drift in feet
Ws = cross wind component speed in ft/sec
Tlag = lag time in seconds as defined above.
This isn't complicated. Most ballistics programs will give time of flight in air. The time of flight in a vacuum is the distance in feet divided by the MV in ft/sec. For example, 300 feet /3,000 ft/sec = 0.1 second flight time to 100 yards in a vacuum for a bullet with 3,000 ft/sec muzzle velocity.
BC gets into the act because it represents the physical properties (weight, cross sectional area, and form factor) that cause the lag time. The way BC is defined a higher BC bullet will have shorter lag time and thus less wind drift. BC is a convenient one number index of how well the bullet will penetrate the air with out losing velocity.
Works for me anyway.
You could get that info in the 4th edition of the Sierra reloading manual years ago. What is your point? We all know that windage is partly based on TOF however, TOF in and of itself is not what makes for wind drift. In other word, the shorter the TOF doesnt always equate to less drift.
I may be way off base here but it seems as if you dont agree with Jon A. Jon A is sopt on here. You are too.
Comparing time in a vacume versus time in the apmosphere is how you find wind drift in inches yes but does little if any to show you the effects of a high BC bullet at low velocity versus a low BC bullet at high velcoity. The example below shows that the lag time for both is nearly identical. What the lag time DOES NOT show you is how fast a bullet started and what it's BC was. It is simply 2 numbers one of which is subtracted from the other.
A bullet with a very short TOF at 1K may have a TOF of 1.2300 seconds where another may have a TOF of 1.5500 seconds. If the bullet that has 1.2300 second TOF is a 168 SMK at 3600 FPS at the muzzle and the bullet that has a 1.5500 second TOF is a 208 AMAX at 2600 FPS at the muzzle, guess which one will drift less. Well.......................... What is your guess?
Scroll down for the answer
Did you guess the 168 SMK?? If you did then you are wrong.
Even though the 168 got there 0.32 seconds faster at 1K, the 208 actually arrives with a couple less inches of drift despite exiting the muzzle at 1000 FPS less than the 168.
How can that be? Because the 'lag time' of each was near identical even though one bullet was very fast and one was very slow. That is the point. This is how a high BC bullet can have the same drift at slower speeds than a very fast low BC bullet. It is NOT the TOF that determines the wind drift rather how slowly or fast the bullet slowed down over the course of it's time in the air.
The reason low BC bullets at super high velocities drop less yet drift more than slow high BC bullets is because drop is a function of 2 principals. 1: Air drag and 2: gravity. The shorter the TOF, the further it will travel before hitting the ground which is VERY importand for a flat trajectory whereas with windage, sure TOF is a factor for windage but not near as much a factor as is for drop. Gravity is not a major component in windage like it is drop. The idea isnt to get a super short TOF for windage, the idea is to use a heavy bullet with a good form factor even if the TOF is not impresive. The high BC bullet WILL drift less than a low BC bullet even if they are 1000 FPS different on the muzzle end. Have you noticed that when you run the numbers using TOF and TOF in a vacume for your 'lag' time that niether factor in gravity?