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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: 479669" data-attributes="member: 28191"><p>I wouldn't say that a BC 20% higher than expected is impossible, but rather that extraordinary claims require extraordinary evidence to back them up. A small minority of bullets have G7 BCs that exceed their sectional density, and most of these only exceed the sectional density 5% or less. Therefore, any claim of a G7 BC that is more than the sectional density (or a G1 BC more than twice the sectional density) is an extraordinary claim, and I would tend to be doubtful unless and until the claim is verified by an independent reliable source that is capable of measuring BCs with either dual chronographs or a time of flight method. </p><p></p><p>It is an astute observation that most of the ways that an imperfect barrel or misformed bullet change the BC is to lower it by increased drag. There are a small number of cases where we've measured BCs 10% or so higher than the bullet manufacturer's claims (Litz also), but we've never had a case where a majority of rifles we tested had consistent BCs and one rifle gave a much higher BC than the rest. The odd rifle out seems to always give a lower BC. </p><p></p><p>The different hypotheses I mentioned above regarding why rifles can differ in BC are all viable and difficult to rule out in any specific case. In the absence of carefully designed and executed experiments that minimize confounding effects, we'd only be guessing. It may well be a crown defect inducing a large yaw early in flight, but any crown defect has gone unnoticed by myself and a skilled local gunsmith. The most compelling verification of that hypothesis would be to have the rifle recrowned and then retest the BC and have it fall into line with the other rifles. However, I'm about at the end of my patience with that barrel and I think I'll have a 6.5-284 Lilja barrel put on that rifle to serve as a 600 yard critter smacker and 1000 yard paper puncher if it shoots really well. My daughters (12 and 14) both want to shoot my 25-06 Sendero, and it would be nice to have a similar rig with the higher BCs of the 140 grain bullets.</p><p></p><p>I think it is a mistake to allocate too much of one's shot placement error budget to uncertainties in BC. Sure, lowering the BC by 13% might not produce enough additional drop to cause a miss, but a lot of other factors contribute to inaccuracies. I prefer that controllable factors like BC not be adding to my shot placement errors. Several of the articles on bullet pointing and meplat trimming to bring BC consistency into the 1% range make a lot of sense to me. Of course, so do aluminum and plastic tipped bullets for better BC consistency. A lower than expected BC not only produces more drop than expected, it produces more wind drift, which produces sub-optimal shot placement. It also produces lower impact velocity than expected which runs the risk of falling below the bullet's window for reliable expansion. Finally, a lower than expected BC can make the sub-sonic transition much closer than expected.</p><p></p><p>I don't know of any program that determines BCs from downrange impacts, as the only methods generally considered valid are the dual chronograph method measuring velocity loss over a fixed distance and the method of using a near chronograph and time of flight. Using point of impact, one can only proceed by guessing different BCs and then assuming you are close when the downrange point of impact matches. Of course, this approach assumes that BC is the only variable factor in downrange point of impact and that all the possible confounding factors are perfectly known and controlled, because the point of impact method is much more susceptible to a much broader variety of errors. Small variations in muzzle velocity, inherent accuracy, up drafts, down drafts, air density, scope issues, and even the Coriolis effect negatively impact this approach.</p><p></p><p>On the whole, BCs are a useful tool for predicting wind drift, retained energy, and bullet drop once a BC is determined for a given rifle. A BC from a bullet manufacturer or measured by another shooter in a different rifle might only be useful for getting on the paper at long range or for determining a short list of the most promising bullet candidates to try in a new rifle project. Experience and some analysis of the published Litz data also gives some clues as to which bullet designs are more commonly associated with exaggerated BC claims from their manufacturers.</p><p></p><p>Michael</p></blockquote><p></p>
[QUOTE="Michael Courtney, post: 479669, member: 28191"] I wouldn't say that a BC 20% higher than expected is impossible, but rather that extraordinary claims require extraordinary evidence to back them up. A small minority of bullets have G7 BCs that exceed their sectional density, and most of these only exceed the sectional density 5% or less. Therefore, any claim of a G7 BC that is more than the sectional density (or a G1 BC more than twice the sectional density) is an extraordinary claim, and I would tend to be doubtful unless and until the claim is verified by an independent reliable source that is capable of measuring BCs with either dual chronographs or a time of flight method. It is an astute observation that most of the ways that an imperfect barrel or misformed bullet change the BC is to lower it by increased drag. There are a small number of cases where we've measured BCs 10% or so higher than the bullet manufacturer's claims (Litz also), but we've never had a case where a majority of rifles we tested had consistent BCs and one rifle gave a much higher BC than the rest. The odd rifle out seems to always give a lower BC. The different hypotheses I mentioned above regarding why rifles can differ in BC are all viable and difficult to rule out in any specific case. In the absence of carefully designed and executed experiments that minimize confounding effects, we'd only be guessing. It may well be a crown defect inducing a large yaw early in flight, but any crown defect has gone unnoticed by myself and a skilled local gunsmith. The most compelling verification of that hypothesis would be to have the rifle recrowned and then retest the BC and have it fall into line with the other rifles. However, I'm about at the end of my patience with that barrel and I think I'll have a 6.5-284 Lilja barrel put on that rifle to serve as a 600 yard critter smacker and 1000 yard paper puncher if it shoots really well. My daughters (12 and 14) both want to shoot my 25-06 Sendero, and it would be nice to have a similar rig with the higher BCs of the 140 grain bullets. I think it is a mistake to allocate too much of one's shot placement error budget to uncertainties in BC. Sure, lowering the BC by 13% might not produce enough additional drop to cause a miss, but a lot of other factors contribute to inaccuracies. I prefer that controllable factors like BC not be adding to my shot placement errors. Several of the articles on bullet pointing and meplat trimming to bring BC consistency into the 1% range make a lot of sense to me. Of course, so do aluminum and plastic tipped bullets for better BC consistency. A lower than expected BC not only produces more drop than expected, it produces more wind drift, which produces sub-optimal shot placement. It also produces lower impact velocity than expected which runs the risk of falling below the bullet's window for reliable expansion. Finally, a lower than expected BC can make the sub-sonic transition much closer than expected. I don't know of any program that determines BCs from downrange impacts, as the only methods generally considered valid are the dual chronograph method measuring velocity loss over a fixed distance and the method of using a near chronograph and time of flight. Using point of impact, one can only proceed by guessing different BCs and then assuming you are close when the downrange point of impact matches. Of course, this approach assumes that BC is the only variable factor in downrange point of impact and that all the possible confounding factors are perfectly known and controlled, because the point of impact method is much more susceptible to a much broader variety of errors. Small variations in muzzle velocity, inherent accuracy, up drafts, down drafts, air density, scope issues, and even the Coriolis effect negatively impact this approach. On the whole, BCs are a useful tool for predicting wind drift, retained energy, and bullet drop once a BC is determined for a given rifle. A BC from a bullet manufacturer or measured by another shooter in a different rifle might only be useful for getting on the paper at long range or for determining a short list of the most promising bullet candidates to try in a new rifle project. Experience and some analysis of the published Litz data also gives some clues as to which bullet designs are more commonly associated with exaggerated BC claims from their manufacturers. Michael [/QUOTE]
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