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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
MERC - Maximum Effective Range Calculator
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<blockquote data-quote="entoptics" data-source="post: 1968252" data-attributes="member: 104268"><p>To build on [USER=112986]@speedengineer[/USER]'s last post...</p><p></p><p>How does wind direction affect hit probability? We often assume that a tail wind is ideal, as it has a "zero" value (ignoring resistance and jump affects, which are generally small anyway). This is true if there's no uncertainty in the wind direction. As soon as the wind direction is variable, the tail/head wind can actually cause more dispersion and lower hit probability than a full value wind...</p><p></p><p>Here's a couple of simulations with a 7mm Mag, with a 0.7 B.C. bullet at 2920 fps, so it's a stellar wind performer.</p><p></p><p>10 ± 3 mph wind, 30° uncertainty in wind direction, run for "zero value" tail wind (180°) and a "full value" crosswind (90°). Notice that the 90° wind has a 10% better hit percentage!</p><p></p><p>[ATTACH=full]210981[/ATTACH]</p><p></p><p>How can that be?!?! </p><p></p><p>Well, though the absolute magnitude of the crosswind is greater, it's important to remember that we're adjusting for that with our hold or dials. What MERC simulates is the <em>variation </em>in the wind due to our uncertainty in the wind direction and wind velocity.</p><p></p><p>So to explain the phenomenon in more detail...</p><p></p><p>The black arrows are the spread in the wind direction, and the dashed red arrows are the 90° component that actually affects the bullet.</p><p></p><p>As you can see, the full variation in wind affect is relatively small over large swings in direction for crosswinds.</p><p>[ATTACH=full]210992[/ATTACH]</p><p></p><p>With a tail wind, relatively small changes in wind angle result in larger variation in the crosswind component, and even will switch the direction of the wind correction!</p><p>[ATTACH=full]210995[/ATTACH]</p><p></p><p>So, when we look at the hits on our 500 yard target, it should make sense. </p><p></p><p>With a variable tail wind (or head wind of course), you've made no correction for the windage, so if the wind shifts away from directly behind you, there's now a horizontal dispersion, that can switch depending on which way the wind changes. This shows in the much larger horizontal dispersion in the hit locations.</p><p></p><p>With a variable crosswind, the maximum and minimum windage change is far less, so the hits won't be spread as widely. One thing to notice though...The hits are not quite centered around the bullseye. They favor the right side a little bit (6 missed to the right, and only one missed to the left). This is another neat phenomenon that we'll address in another post soon...</p></blockquote><p></p>
[QUOTE="entoptics, post: 1968252, member: 104268"] To build on [USER=112986]@speedengineer[/USER]'s last post... How does wind direction affect hit probability? We often assume that a tail wind is ideal, as it has a "zero" value (ignoring resistance and jump affects, which are generally small anyway). This is true if there's no uncertainty in the wind direction. As soon as the wind direction is variable, the tail/head wind can actually cause more dispersion and lower hit probability than a full value wind... Here's a couple of simulations with a 7mm Mag, with a 0.7 B.C. bullet at 2920 fps, so it's a stellar wind performer. 10 ± 3 mph wind, 30° uncertainty in wind direction, run for "zero value" tail wind (180°) and a "full value" crosswind (90°). Notice that the 90° wind has a 10% better hit percentage! [ATTACH type="full"]210981[/ATTACH] How can that be?!?! Well, though the absolute magnitude of the crosswind is greater, it's important to remember that we're adjusting for that with our hold or dials. What MERC simulates is the [I]variation [/I]in the wind due to our uncertainty in the wind direction and wind velocity. So to explain the phenomenon in more detail... The black arrows are the spread in the wind direction, and the dashed red arrows are the 90° component that actually affects the bullet. As you can see, the full variation in wind affect is relatively small over large swings in direction for crosswinds. [ATTACH type="full"]210992[/ATTACH] With a tail wind, relatively small changes in wind angle result in larger variation in the crosswind component, and even will switch the direction of the wind correction! [ATTACH type="full"]210995[/ATTACH] So, when we look at the hits on our 500 yard target, it should make sense. With a variable tail wind (or head wind of course), you've made no correction for the windage, so if the wind shifts away from directly behind you, there's now a horizontal dispersion, that can switch depending on which way the wind changes. This shows in the much larger horizontal dispersion in the hit locations. With a variable crosswind, the maximum and minimum windage change is far less, so the hits won't be spread as widely. One thing to notice though...The hits are not quite centered around the bullseye. They favor the right side a little bit (6 missed to the right, and only one missed to the left). This is another neat phenomenon that we'll address in another post soon... [/QUOTE]
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MERC - Maximum Effective Range Calculator
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