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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Let's argue about BC's
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<blockquote data-quote="Michael Courtney" data-source="post: 480475" data-attributes="member: 28191"><p>Magnitudes of BC Variations with Different Rifles and Lot Numbers</p><p></p><p>In my last post, I discussed published data (Litz) showing that a single box of bullets can exhibit shot-to-shot variations in BC above 10%. We've seen the same thing in both our published and unpublished data. Sometimes a single box of bullets exhibits small (1-2%) shot-to-shot variations in BC; sometimes the shot-to-shot variations are much larger (10-16%).</p><p></p><p>Published data where BC is presented for several different rifles and the same lot of bullets is harder to come by. Likewise, I don't know of much published data for the same rifle measuring the BC for several different lots of bullets manufactured years apart to determine with confidence how much the BC of a given model of bullet might be changing over time. The best we can infer from published data includes effects of both different rifles and different lot numbers.</p><p></p><p>Before comparing data from different sources to speak to this question, care is needed to ensure one has a high level of confidence in the accuracy of the published numbers from the sources, as well as to ensure that both sources are stating their BC for the same velocity, since we know that BC can exhibit significant changes with velocity. Here, we'll discuss variations between Bryan Litz's and Sierra's published numbers for their .224 and .308 caliber bullets where both have published BCs at 3000 fps. Since in most cases, Sierra published their numbers many years before Bryan, I think it is safe to say that in most (if not all) cases, Sierra's numbers result from testing different lot numbers in different rifle barrels from Bryan's tests.</p><p></p><p>In the .224 caliber bullets for which they both report G1 BC measurements, Bryan reports a BC measurement 14% higher (0.350) than Sierra (0.301) for the 69 grain SMK. In contrast, Bryan reports a BC measurement 3% lower (0.218) than Sierra (0.225) for the 52 grain SMK. Measurements for the other .224 caliber bullets are closer.</p><p></p><p>Similar variations in reported BCs exist for the .308 caliber bullets for which they both report G1 BC measurements good at 3000 fps. Bryan Litz reports a BC measurement 10% higher (0.448) than Sierra (0.404) for the 165 grain GameKing at 3000 fps. In contrast, Bryan reports a BC measurement 11% lower (0.639) than Sierra (0.711) for the 240 SMK.</p><p></p><p>We have not measured as wide a variety of bullets as Bryan Litz or Sierra bullets, but our (published and unpublished) BCs suggest that variations between different rifles (same lot of bullets) and variations between different lots (same rifle) can both exceed 10%.</p><p></p><p>Michael</p></blockquote><p></p>
[QUOTE="Michael Courtney, post: 480475, member: 28191"] Magnitudes of BC Variations with Different Rifles and Lot Numbers In my last post, I discussed published data (Litz) showing that a single box of bullets can exhibit shot-to-shot variations in BC above 10%. We've seen the same thing in both our published and unpublished data. Sometimes a single box of bullets exhibits small (1-2%) shot-to-shot variations in BC; sometimes the shot-to-shot variations are much larger (10-16%). Published data where BC is presented for several different rifles and the same lot of bullets is harder to come by. Likewise, I don't know of much published data for the same rifle measuring the BC for several different lots of bullets manufactured years apart to determine with confidence how much the BC of a given model of bullet might be changing over time. The best we can infer from published data includes effects of both different rifles and different lot numbers. Before comparing data from different sources to speak to this question, care is needed to ensure one has a high level of confidence in the accuracy of the published numbers from the sources, as well as to ensure that both sources are stating their BC for the same velocity, since we know that BC can exhibit significant changes with velocity. Here, we'll discuss variations between Bryan Litz's and Sierra's published numbers for their .224 and .308 caliber bullets where both have published BCs at 3000 fps. Since in most cases, Sierra published their numbers many years before Bryan, I think it is safe to say that in most (if not all) cases, Sierra's numbers result from testing different lot numbers in different rifle barrels from Bryan's tests. In the .224 caliber bullets for which they both report G1 BC measurements, Bryan reports a BC measurement 14% higher (0.350) than Sierra (0.301) for the 69 grain SMK. In contrast, Bryan reports a BC measurement 3% lower (0.218) than Sierra (0.225) for the 52 grain SMK. Measurements for the other .224 caliber bullets are closer. Similar variations in reported BCs exist for the .308 caliber bullets for which they both report G1 BC measurements good at 3000 fps. Bryan Litz reports a BC measurement 10% higher (0.448) than Sierra (0.404) for the 165 grain GameKing at 3000 fps. In contrast, Bryan reports a BC measurement 11% lower (0.639) than Sierra (0.711) for the 240 SMK. We have not measured as wide a variety of bullets as Bryan Litz or Sierra bullets, but our (published and unpublished) BCs suggest that variations between different rifles (same lot of bullets) and variations between different lots (same rifle) can both exceed 10%. Michael [/QUOTE]
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