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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Aerodynamic Jump and Ballistic Solutions
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<blockquote data-quote="MMERSS" data-source="post: 864872" data-attributes="member: 63748"><p>First, welcome to Long Range Hunting. I am somewhat surprised you did not ask this question on accurateshooter and insisted on a long range hunting forum. None the less you have asked one of the best questions I have seen in quite some time. Your concerns are very notable and well deserved. Bryan's books are fantastic! His end summaries are right on target, no pun intended.</p><p></p><p>Do I consider vertical deflection caused by the gyration of bullets in a crosswind? In most cases be it hunting or F-Class competition usually not. Here is my explanation of when I would or would not consider the vertical corrections and how I chart the corrections when I do consider them. These are my opinions only based on my shooting habits and others will have their own.</p><p></p><p>I recently developed a load for a 300 RUM using the Berger Hybrid Target bullet. This is a long range shooting and hunting forum so I will use this combination as an example.</p><p></p><p>Data required for calculations:</p><p>Weight: 230gr</p><p>MV: 3171 fps</p><p>OAL: 1.640"</p><p>Bullet diameter: .308"</p><p>Right Twist Barrel</p><p>Twist 1 in 10"</p><p></p><p>Bryan's equation Y=0.01 x Sg-0.0024 x L + 0.032 is dependent on the air density to determine Sg as L is constant (OAL/bullet diameter=1.640/.308=5.32). Asking the question; So what does air density change have to influence Sg as used in this equation? I will use a density altitude of 0 feet and one of 6000 feet for comparison to answer this question.</p><p></p><p>Using a density altitude of 0 feet and an analyzer using the Miller/Courtney method Sg=1.49.</p><p>For a density altitude of 6000 feet Sg=1.79.</p><p>Both altitudes should insure stability.</p><p></p><p>Using Bryan's equation for each air density:</p><p>DA 0ft: Y=.01 x 1.49 – 0.0024 x 5.32 + .032 = .034</p><p>DA 6000ft: Y= .01 x 1.79 – 0.0024 x 5.32 + .032 = .037</p><p></p><p>Back to the question; So what does air density change have to influence Sg as used in this equation? Lets assume we are shooting in a 10 MPH cross wind. Vertical correction in a 0ft DA air density equivalent is .34 MOA. Vertical correction in a 6000ft DA air density equivalent is .37MOA. The difference is .03MOA. I'm not aware of any scope capable of making corrections this small. In other words, pick average atmospheric conditions you will be shooting in and calculate your Y. The majority of my scopes have ¼ MOA corrections. The closest conversion is 1 click (.25 MOA) for the .34 MOA correction for a 10 MPH crosswind, 3 clicks for a 20 MPH crosswind (.34 + .34 = .68 i.e. .75 MOA correction is closest), etc.</p><p></p><p>So what does all this mean to the long range hunter? I limit my maximum range big game hunting shots based off cross wind condition confidence. If I can estimate the crosswind to within 1 MPH my max range with the system above is 1200 yards. 2 MPH 875 yards. 3 MPH 700 yards. How I established these limits is a completely different topic but none the less they serve a purpose here.</p><p></p><p>I am fortunate if I'm shooting in a 10 MPH full value crosswind to be within 2MPH of actual value when I pull the trigger. This means my max range is 875. In all practicality I will be shooting at an elk sized target at this established maximum effective range. Let's say I do not correct for the vertical correction due to bullet gyration. Approximately how many inches of vertical error am I not accounting for? Just less than three. My muzzle velocity variation and terrain changes will probably produce more error than the vertical correction not accounted for.</p><p></p><p>However, for your situation where you are more than likely shooting in crosswinds approaching or exceeding 10MPH most of the time and you will be shooting extended distances, simply apply your Y calculation. In my example the average is .35MOA for each 10MPH crosswind. Round to the nearest 1/4MOA and click down for a 90 deg wind and up for a 270 deg wind. You can easily write .35MOA with arrows pointing up/down-wind left/right in the corner of your firing charts indicating a 10MPH crosswind vertical correction. Again this number is an approximation of your average air density and used independent of air density changes due to the little value of the change.</p></blockquote><p></p>
[QUOTE="MMERSS, post: 864872, member: 63748"] First, welcome to Long Range Hunting. I am somewhat surprised you did not ask this question on accurateshooter and insisted on a long range hunting forum. None the less you have asked one of the best questions I have seen in quite some time. Your concerns are very notable and well deserved. Bryan’s books are fantastic! His end summaries are right on target, no pun intended. Do I consider vertical deflection caused by the gyration of bullets in a crosswind? In most cases be it hunting or F-Class competition usually not. Here is my explanation of when I would or would not consider the vertical corrections and how I chart the corrections when I do consider them. These are my opinions only based on my shooting habits and others will have their own. I recently developed a load for a 300 RUM using the Berger Hybrid Target bullet. This is a long range shooting and hunting forum so I will use this combination as an example. Data required for calculations: Weight: 230gr MV: 3171 fps OAL: 1.640” Bullet diameter: .308” Right Twist Barrel Twist 1 in 10” Bryan’s equation Y=0.01 x Sg-0.0024 x L + 0.032 is dependent on the air density to determine Sg as L is constant (OAL/bullet diameter=1.640/.308=5.32). Asking the question; So what does air density change have to influence Sg as used in this equation? I will use a density altitude of 0 feet and one of 6000 feet for comparison to answer this question. Using a density altitude of 0 feet and an analyzer using the Miller/Courtney method Sg=1.49. For a density altitude of 6000 feet Sg=1.79. Both altitudes should insure stability. Using Bryan’s equation for each air density: DA 0ft: Y=.01 x 1.49 – 0.0024 x 5.32 + .032 = .034 DA 6000ft: Y= .01 x 1.79 – 0.0024 x 5.32 + .032 = .037 Back to the question; So what does air density change have to influence Sg as used in this equation? Lets assume we are shooting in a 10 MPH cross wind. Vertical correction in a 0ft DA air density equivalent is .34 MOA. Vertical correction in a 6000ft DA air density equivalent is .37MOA. The difference is .03MOA. I’m not aware of any scope capable of making corrections this small. In other words, pick average atmospheric conditions you will be shooting in and calculate your Y. The majority of my scopes have ¼ MOA corrections. The closest conversion is 1 click (.25 MOA) for the .34 MOA correction for a 10 MPH crosswind, 3 clicks for a 20 MPH crosswind (.34 + .34 = .68 i.e. .75 MOA correction is closest), etc. So what does all this mean to the long range hunter? I limit my maximum range big game hunting shots based off cross wind condition confidence. If I can estimate the crosswind to within 1 MPH my max range with the system above is 1200 yards. 2 MPH 875 yards. 3 MPH 700 yards. How I established these limits is a completely different topic but none the less they serve a purpose here. I am fortunate if I’m shooting in a 10 MPH full value crosswind to be within 2MPH of actual value when I pull the trigger. This means my max range is 875. In all practicality I will be shooting at an elk sized target at this established maximum effective range. Let’s say I do not correct for the vertical correction due to bullet gyration. Approximately how many inches of vertical error am I not accounting for? Just less than three. My muzzle velocity variation and terrain changes will probably produce more error than the vertical correction not accounted for. However, for your situation where you are more than likely shooting in crosswinds approaching or exceeding 10MPH most of the time and you will be shooting extended distances, simply apply your Y calculation. In my example the average is .35MOA for each 10MPH crosswind. Round to the nearest 1/4MOA and click down for a 90 deg wind and up for a 270 deg wind. You can easily write .35MOA with arrows pointing up/down-wind left/right in the corner of your firing charts indicating a 10MPH crosswind vertical correction. Again this number is an approximation of your average air density and used independent of air density changes due to the little value of the change. [/QUOTE]
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