Posted: 2/5/2014 4:09:09 PM EDT
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I was at a match last week where the competitor next to me said he measured the wind at 6 MPH 1/4 value and adjusted his rear sight 2 MOA. After shooting, he said he'd be reducing his offset .5 MOA for each 100 yards we moved forward. I told him that didn't make sense to me as the angle is the same no matter what the distance. If you're closer, that's just less time for the wind to impart drift on your bullet. He then tried to explain the math, but my brain shut down and I didn't really hear anything he said.
Was his strategy correct? |
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Quoted:
I was at a match last week where the competitor next to me said he measured the wind at 6 MPH 1/4 value and adjusted his rear sight 2 MOA. After shooting, he said he'd be reducing his offset .5 MOA for each 100 yards we moved forward. I told him that didn't make sense to me as the angle is the same no matter what the distance. If you're closer, that's just less time for the wind to impart drift on your bullet. He then tried to explain the math, but my brain shut down and I didn't really hear anything he said. Was his strategy correct? Sir, if I understand the question you're asking if a quarter value of a 6MPH wind is two MOA sight adjustment. On my rifles each click is 1/4 of an MOA so a two MOA adjustment would be eight clicks. Typical wind charts that I have seen show that the recommended adjustment for a quarter value 6MPH wind would be one half to 3/4 MOA at 300yd and about double the adjustment at 600yd. Regardless the mathematical explanation seldom have I seen a condition that is constant enough to last through a twenty shot string. I would say your friends 2MOA sight adjustment was excessive and would expect his POI on paper to reflect such. Regardless, after he made such an adjustment his first shot should have been an indication of his over estimation of the wind effect on the bullet POI. Not sure if I've been very clear, but HTH. 7zero1 out. |
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No, sorry, let me simplify it.
If a shooter uses a 2 MOA offset at 400 yards for a given wind condition, should that same 2 MOA offset also be used at shorter distances if the wind maintained the same speed and direction? This shooter claimed the MOA offset needed to be reduced as he got closer to the target. |
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Quoted: No, sorry, let me simplify it. If a shooter uses a 2 MOA offset at 400 yards for a given wind condition, should that same 2 MOA offset also be used at shorter distances if the wind maintained the same speed and direction? This shooter claimed the MOA offset needed to be reduced as he got closer to the target. It might need to be, the closer to the target, the less time the wind has to affect the projectile. Been a while since I shot highpower, though. |
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That would sound about right.
For wind that is perpendicular to the path of your bullet, the simple wind rule is: 1MOA, per 10 mph of wind, per 100m In this case, given 6.4mph at 400 meters .64 x 400 = 2.56 MOA The further out the bullet has to travel, the more the wind can act on it. Furthermore, the bullet loses velocity as it travels it's path, so at extreme ranges, wind can push the bullet farther than it did at closer ranges. |
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I downloaded a Winchester ballistics calculator app. Here's the drift for a 62 grain .223 round in a 6 MPH full value wind. I converted the drift to MOA at the given distance.
000 yds 00.0 in 0.0 MOA 100 yds 00.5 in 0.5 MOA 200 yds 02.1 in 1.1 MOA 300 yds 04.9 in 1.6 MOA 400 yds 09.3 in 2.3 MOA 500 yds 15.6 in 3.1 MOA So if my math is correct, you really do have to reduce your offset as you get closer (or increase it was you move back). 
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Quoted:
Hope this helps. I'm a visual learner, so that actually helps a lot. Now that I see how it works, I want to know why it works that way. I can totally buy into Ken914's statement that as the bullet loses velocity, it takes more time to go each 100 yards, so therefore the wind has more time to act upon the bullet. |
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You don't have to get too deep in the weeds to know that you should adjust your sights based on current shooting conditions. Do you have a wind chart for your bullet? Use that as a reference. (I can send you one for 75gr 77gr or 80gr if you don't have one.)
I have heard two ways of thinking about the wind. One is that the amount of bullet deflection near your firing point has a greater magnification on how far the bullet is of course as it goes down range. If the wind blows strong at the 600yd line, a 1 degree shift will be a much bigger shift down range than if the wind blew it off 1 degree at the 200yd line. The other thought is that the bullet "slows" as it goes down range and therefore the wind has a little more time to act on it. This means that the bullet will cut through the wind at the 600yd line and be blown off course as it flies down range. I haven't decided which is more correct or which is more important. The bottom line is that you need to correct a certain amount at each range for the given conditions. These corrections have already been calculated and recalculated for high power and have been organized in handy wind charts. Use the wind chart to get close. Use your data book and experience reading the wind to get better. I try to read the mirage for the prevailing wind, watch down range conditions for changes in the wind, and pay attention to the wind on my position before I shoot. If the mirage is consistent, and the flags are consistent, I adjust my sights and try to fire with the wind doing whatever it was doing. If it lets off or picks up I try to wait. This is a lot harder than it sounds, because you are usually focused on the target, score, sights, position, or whatever. You have to learn to put all that on auto-pilot to really watch the wind. It may help to have someone spot for you and have them make wind calls silently. Compare what you thought the wind was doing when you shot to what they saw. They will be much more focused on the wind and may help you recognize shifts and adjustments. |