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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
G7 > G1 ; resistance to BC change
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<blockquote data-quote="Mikecr" data-source="post: 505691" data-attributes="member: 1521"><p>YOUR particular bullet's BC is tied to IT'S particular drag at every velocity. Any G1 BC rating for your bullet is an approximate correlation to the G1 standard bullet drag model.</p><p>Throughout the velocity range of bullet flight, differences between your actual BCs and those calculated based on G1 cause prediction errors. </p><p>Most software utilizes an entered G1BC, throughout bullet travel, as though this is correct. Afterall, you entered it.</p><p>But if your bullet's drag doesn't match the G1 curve at every velocity downrange, then the local/incremental/software BC would likely be wrong. Well, right at some point in travel, and wrong everywhere else.</p><p>So if you adjust your BC for a match at a particular range, while that BC is actually erroneous about that point, you'll be most accurate ONLY at that specific range/BC match.</p><p></p><p>For instance, let's say you validate that .600 ICAO G1BC is tits at 1000yds, with an JLK boat tail bullet. Let's also say that the G1 curve does not accurately represent a drag curve for your bullet. What you would likely find with further testing at various ranges, is that the 1000yd validated BC produces fairy poor matches in POI to predicted, at both near ranges, and those beyond 1000. That results are best only at 1000yds(as validated).</p><p></p><p>A 190JLK with with a G1BC of .600, launched at 2845fps would be traveling 1500fps at a 1Kyd target. It's G1BC is:</p><p>.600 at the muzzle</p><p>.590 at 100yds</p><p>.578 at 500yds</p><p>.532 at 1000yds </p><p>This is an 11% deviation for that range.</p><p></p><p>Bryan Litz considered this error, built into today's mismatch between popular LR bullet drag and the G1 standard. He bracketed this error into a window, and adjusted Berger's BC to reduce (overall)deviations from predicted POI while using G1 BCs.</p><p>He chose BCs based on a more median velocity, rather than muzzle velocity.</p><p>This resulted in lower BCs, but also lower errors in the field.</p><p></p><p>BUT, the G7 drag standard matches the 190JLK bullet better:</p><p>.298 at the muzzle</p><p>.296 at 100yds</p><p>.290 at 500yds</p><p>.301 at 1000yds</p><p>This is a 4% deviation for that range. And Bryan has supported use of G7 as much as anyone.</p><p></p><p>Different methods are affected by this differently. Pejsa approach bypasses it, and is applied well in Loadbase. But nothing would be more accurate than incremental application of an actual matching drag curve.</p><p>Anyway, hopefully this helps with understanding.</p></blockquote><p></p>
[QUOTE="Mikecr, post: 505691, member: 1521"] YOUR particular bullet's BC is tied to IT'S particular drag at every velocity. Any G1 BC rating for your bullet is an approximate correlation to the G1 standard bullet drag model. Throughout the velocity range of bullet flight, differences between your actual BCs and those calculated based on G1 cause prediction errors. Most software utilizes an entered G1BC, throughout bullet travel, as though this is correct. Afterall, you entered it. But if your bullet's drag doesn't match the G1 curve at every velocity downrange, then the local/incremental/software BC would likely be wrong. Well, right at some point in travel, and wrong everywhere else. So if you adjust your BC for a match at a particular range, while that BC is actually erroneous about that point, you'll be most accurate ONLY at that specific range/BC match. For instance, let's say you validate that .600 ICAO G1BC is tits at 1000yds, with an JLK boat tail bullet. Let's also say that the G1 curve does not accurately represent a drag curve for your bullet. What you would likely find with further testing at various ranges, is that the 1000yd validated BC produces fairy poor matches in POI to predicted, at both near ranges, and those beyond 1000. That results are best only at 1000yds(as validated). A 190JLK with with a G1BC of .600, launched at 2845fps would be traveling 1500fps at a 1Kyd target. It's G1BC is: .600 at the muzzle .590 at 100yds .578 at 500yds .532 at 1000yds This is an 11% deviation for that range. Bryan Litz considered this error, built into today's mismatch between popular LR bullet drag and the G1 standard. He bracketed this error into a window, and adjusted Berger's BC to reduce (overall)deviations from predicted POI while using G1 BCs. He chose BCs based on a more median velocity, rather than muzzle velocity. This resulted in lower BCs, but also lower errors in the field. BUT, the G7 drag standard matches the 190JLK bullet better: .298 at the muzzle .296 at 100yds .290 at 500yds .301 at 1000yds This is a 4% deviation for that range. And Bryan has supported use of G7 as much as anyone. Different methods are affected by this differently. Pejsa approach bypasses it, and is applied well in Loadbase. But nothing would be more accurate than incremental application of an actual matching drag curve. Anyway, hopefully this helps with understanding. [/QUOTE]
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