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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Zero shift on travel to new location.
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<blockquote data-quote="Brown Dog" data-source="post: 58587" data-attributes="member: 1622"><p>Goodgrouper,</p><p></p><p>Wow; just read your link. That'll be a part of the answer /ubbthreads/images/graemlins/smile.gif!</p><p></p><p>I would suspect that -with relatively small latitude changes- it is the gravitational anomalies that are the major player (ie denser rock and the Earth not being a perfect sphere.)But I think the effects would be tiny:</p><p></p><p>I moved just over 6 degrees N between zeroes; Baum's article (your link) indicated gravitational acceleration increasing from 978.9cm/sec/sec at the equator to 982.3 cm/sec/sec at the poles. (Which is interesting because the Brit Standard unit for gravity is 981cm/sec/sec –which about matches our latitude for Baum's values.) </p><p></p><p>Some very broad bush thinking: With Baum's values if all other conditions remained the same (pretty difficult to do!), a projectile with a 1 second time of flight would impact 3.4cm (1.3in) lower at the pole than the equator. So at 100m (0.13 sec TOF for my 308) we would see a 0.44cm (0.17in) change of impact between equator and pole –not too significant! –particularly when you consider that I only moved a little over 6 degrees N and would therefore see only a tiny fraction of that equator to pole change.</p><p></p><p>What, I suspect, would be more significant would be the gravitational anomaly side of life. I live on chalk downland (pretty low density) the Scottish Highlands are one huge lump of granite (pretty dense!).</p><p></p><p>However I still think that the values would be pretty tiny at 100m (probably unmeasurable?). </p><p></p><p>I think a stronger effect would have to be the cause of 0.5 to 0.75 MOA vertical zero shifts at 100m (such as Shawn's vertical wind component idea). </p><p></p><p>What do you think? (is my thinking flawed? -I did a year-long course with a significant ballistics component about 7 years ago, but it's all very rusty to me now!)?</p><p></p><p>I have no access to the Pejsa article. What sort of values does he reckon you would see?</p><p></p><p>And now that we've entered full-on 'ballistic nerd-itry' where's Dave King's input /ubbthreads/images/graemlins/grin.gif?</p></blockquote><p></p>
[QUOTE="Brown Dog, post: 58587, member: 1622"] Goodgrouper, Wow; just read your link. That'll be a part of the answer [img]/ubbthreads/images/graemlins/smile.gif[/img]! I would suspect that -with relatively small latitude changes- it is the gravitational anomalies that are the major player (ie denser rock and the Earth not being a perfect sphere.)But I think the effects would be tiny: I moved just over 6 degrees N between zeroes; Baum’s article (your link) indicated gravitational acceleration increasing from 978.9cm/sec/sec at the equator to 982.3 cm/sec/sec at the poles. (Which is interesting because the Brit Standard unit for gravity is 981cm/sec/sec –which about matches our latitude for Baum’s values.) Some very broad bush thinking: With Baum’s values if all other conditions remained the same (pretty difficult to do!), a projectile with a 1 second time of flight would impact 3.4cm (1.3in) lower at the pole than the equator. So at 100m (0.13 sec TOF for my 308) we would see a 0.44cm (0.17in) change of impact between equator and pole –not too significant! –particularly when you consider that I only moved a little over 6 degrees N and would therefore see only a tiny fraction of that equator to pole change. What, I suspect, would be more significant would be the gravitational anomaly side of life. I live on chalk downland (pretty low density) the Scottish Highlands are one huge lump of granite (pretty dense!). However I still think that the values would be pretty tiny at 100m (probably unmeasurable?). I think a stronger effect would have to be the cause of 0.5 to 0.75 MOA vertical zero shifts at 100m (such as Shawn's vertical wind component idea). What do you think? (is my thinking flawed? -I did a year-long course with a significant ballistics component about 7 years ago, but it’s all very rusty to me now!)? I have no access to the Pejsa article. What sort of values does he reckon you would see? And now that we’ve entered full-on 'ballistic nerd-itry’ where’s Dave King’s input [img]/ubbthreads/images/graemlins/grin.gif[/img]? [/QUOTE]
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