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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Ballistics Question
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<blockquote data-quote="BryanLitz" data-source="post: 328025" data-attributes="member: 7848"><p>jeffro,</p><p>Each of the bullets you mentioned is available in the JBM library with a 'Litz' measured BC. When you choose that bullet, the measured G7 BC for that bullet is loaded which, next to doplar radar data, is the most accurate and best fitting BC for long range bullets.</p><p></p><p>My measured BC's are typically good enough to predict trajectories with less than .5 MOA of error out to 700/800 yards. I suspect the ~.5 MOA error you're seeing is mostly due to imperfections in scope adjustment, or uncertainties in some of the other inputs.</p><p></p><p>How many shots are you shooting to determine the group center in relation to your aim point?</p><p></p><p>To put it in perspective, .5 MOA error in predicted drop isn't very bad at 700/800 yards.</p><p></p><p>It would only take a 2.5% calibration error in your scope's reticle movement to create the .5 MOA error at 800 yards. If you dial 20 MOA and the scope moves 19.5 or 20.5 MOA, you should be able to detect that with the 'tall target' test.</p><p></p><p>To answer your original question: how to fix it?...</p><p>There's no easy way in JBM. Let's say you do the tall target test and find that your reticle moves .256 MOA per click instead of .250 MOA per click. You would have to manually convert the drop table from inches to clicks based on the correction factor that's specific to your scope. This can be done in a spreadsheet.</p><p></p><p>Just to cover this base; are you using the correct click value in JBM? If you selected 1/4" clicks vs 1/4 MOA clicks (or vice versa) that would be more than enough to cause your perceived .5 MOA error at 800 yards.</p><p></p><p>-Bryan</p></blockquote><p></p>
[QUOTE="BryanLitz, post: 328025, member: 7848"] jeffro, Each of the bullets you mentioned is available in the JBM library with a 'Litz' measured BC. When you choose that bullet, the measured G7 BC for that bullet is loaded which, next to doplar radar data, is the most accurate and best fitting BC for long range bullets. My measured BC's are typically good enough to predict trajectories with less than .5 MOA of error out to 700/800 yards. I suspect the ~.5 MOA error you're seeing is mostly due to imperfections in scope adjustment, or uncertainties in some of the other inputs. How many shots are you shooting to determine the group center in relation to your aim point? To put it in perspective, .5 MOA error in predicted drop isn't very bad at 700/800 yards. It would only take a 2.5% calibration error in your scope's reticle movement to create the .5 MOA error at 800 yards. If you dial 20 MOA and the scope moves 19.5 or 20.5 MOA, you should be able to detect that with the 'tall target' test. To answer your original question: how to fix it?... There's no easy way in JBM. Let's say you do the tall target test and find that your reticle moves .256 MOA per click instead of .250 MOA per click. You would have to manually convert the drop table from inches to clicks based on the correction factor that's specific to your scope. This can be done in a spreadsheet. Just to cover this base; are you using the correct click value in JBM? If you selected 1/4" clicks vs 1/4 MOA clicks (or vice versa) that would be more than enough to cause your perceived .5 MOA error at 800 yards. -Bryan [/QUOTE]
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