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View Full Version : How does a tailwind affect zero?


Shampoo
03-18-2007, 3:06 PM
Dudes,

Went out to Ocotillo today to zero my new M1A. Totally psyched after a mind numbing (and hot!) break in session last weekend in absolutely no wind whatsoever. Much to my dismay today though, the wind was a-howlin at 9AM. Not wasting the hour and a half drive, no way...I gutted it out and was able to set up a spot where the wind was coming from my direct 6 o'clock on to the target. I was zeroing an Aimpoint ML2, prone on a sand bag. I was getting 2-4MOA at 100 yards with sand stinging at my ears and eyes before the afternoon gale got to be just too ridiculous. My question is on some a calmer day in the future, where could I reasonbly expect my POI to be under the current settings? Does the tail wind lift the .308 bullet significantly? Make it go down at all? No effect (doubtful)?? It was literally directly behind me to the target so no cross wind. It wasn't no breeze though - it was a mighty, mighty wind. What is your experience? Thanks in advance gentlemen. Shampoo.

Wulf
03-18-2007, 4:05 PM
A tail wind should should mimic a bullet that's flying faster; not exactly, but as a first order approximation.

5280 ft/mi times 10 mi/hr is 52800 ft/hour

52800 ft/hour divided by 60 minutes/hour is 880 ft/minute.

880 ft/minute divided by 60 seconds/minute is 14.7 feet/second. That's for a 10 mph wind.

15 fps is within the shot to shot variation in velocity you would expect with match ammo from a bolt gun; probably half or a third of the variation you'd see in a gas gun like your Springfield. So I'd say you'd not have to make any correction or see any differences at 10mph, probably not 20 or 30 mph either, unless you're working at long long ranges where the minute differences would have time to accumulate. But you wouldent be zeroing an Aimpoint if you were doing that kind of work. So I wouldent worry about it at all.

If you play with it in a ballistic calculator you'll find that at 500 yards, 15 fps difference in velocity on a 308 ball equivalent load makes about 8/10's of an inch difference in drop, that's out of 58 or so total inches of drop. Call it 0.16 moa. Here's the calculator if you want to play around. http://www.eskimo.com/~jbm/ballistics/traj_basic/traj_basic.html

kj
03-18-2007, 8:07 PM
A head wind or tail wind should have no effect at 100 yards using a 308. At 600 yards (the farthest I shoot) I give no value to a head or tail wind. This is with an AR15 shooting 80 grain SMKs.

Mr. Mildot
03-18-2007, 8:34 PM
Generally a headwind will cause the bullet to drop faster (b'cause it's slowing down faster) and a tail wind will cause the bullet to carry further because it's not slowing down as quickly.

If you have access to ballsitic software, you can determine the exact value of wind effected drop if you know your bullet weight, ballistic coefficeint, muzzle velocity and sight height. Additional factors to take not of and input into a ballistic program are ambient temperature, altitude, angle of inclination and humidity.

30Cal
03-19-2007, 7:57 AM
A tailwind is going to be insignificant. If you're shooting a 600yds and your range has really large berms between you and the target, you might see some odd things. But not at 100yds. Just shoot straight away (no-wind zero).

Ty

Mute
03-19-2007, 9:17 AM
The way I learned wind effects is with the clock system. A constant is given for a specific wind speed and then a percentage of the constant is used based on the clock direction of the wind:

http://img.photobucket.com/albums/v78/sycopupy/Guns/clip_image001.jpg

Tailwind is given zero value for the calculation.

Shampoo
03-19-2007, 11:20 AM
thanks folks!