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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
.308 / Berger VLD's of differing weights
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<blockquote data-quote="BryanLitz" data-source="post: 361766" data-attributes="member: 7848"><p><strong>Re: .308 / Berger VLD's of differning weights</strong></p><p></p><p>Mike,</p><p></p><p>The Miller formula is quite accurate, but sometimes the results get mis-interpreted. </p><p></p><p>In theory, SG only needs to be above 1.000 in order for the bullet to be stable. An SG of .999 or less is unstable, SG greater than 1.000 is stable. Because of the potential inherent error in the formula (up to ~10%) and because we shoot in a range of environmental conditions, we add a 'safety margin' to the SG so that we don't get close to 1.000. Some think that a margin of .3 is enough, and advise to keep the SG above 1.3 in nominal conditions so you don't dip down to 1.000. Others advise a safety margin as high as .5, and say you want the SG to be 1.5 or higher in nominal conditions so it doesn't dip down below 1.000. Personally, I advise a safety factor of .4, meaning you should aim for a stability factor of 1.4 in nominal conditions.</p><p></p><p>In other words, the label "under stabilized" that appears in the Loadbase calculation for SG's above 1.3 and 1.4 is very conservative. The only way the bullet won't actually be stable from a 1:12" twist is if there's more than 30% error in the Miller formula which is highly unlikely as 10% is likely to be the max error. If the calculated SG gets below 1.1, I'd start to worry that there might be a problem, but 1.3 and 1.4 is pretty safe.</p><p></p><p>I hope this discussion of the judgment calls involved in safety margins applied to SG helps to ease your mind about shooting the 190's from a 1:12" twist.</p><p></p><p>Take care,</p><p>-Bryan</p><p></p><p>PS, in your shooting this morning, did the 190's group well and make nice round holes?</p></blockquote><p></p>
[QUOTE="BryanLitz, post: 361766, member: 7848"] [b]Re: .308 / Berger VLD's of differning weights[/b] Mike, The Miller formula is quite accurate, but sometimes the results get mis-interpreted. In theory, SG only needs to be above 1.000 in order for the bullet to be stable. An SG of .999 or less is unstable, SG greater than 1.000 is stable. Because of the potential inherent error in the formula (up to ~10%) and because we shoot in a range of environmental conditions, we add a 'safety margin' to the SG so that we don't get close to 1.000. Some think that a margin of .3 is enough, and advise to keep the SG above 1.3 in nominal conditions so you don't dip down to 1.000. Others advise a safety margin as high as .5, and say you want the SG to be 1.5 or higher in nominal conditions so it doesn't dip down below 1.000. Personally, I advise a safety factor of .4, meaning you should aim for a stability factor of 1.4 in nominal conditions. In other words, the label "under stabilized" that appears in the Loadbase calculation for SG's above 1.3 and 1.4 is very conservative. The only way the bullet won't actually be stable from a 1:12" twist is if there's more than 30% error in the Miller formula which is highly unlikely as 10% is likely to be the max error. If the calculated SG gets below 1.1, I'd start to worry that there might be a problem, but 1.3 and 1.4 is pretty safe. I hope this discussion of the judgment calls involved in safety margins applied to SG helps to ease your mind about shooting the 190's from a 1:12" twist. Take care, -Bryan PS, in your shooting this morning, did the 190's group well and make nice round holes? [/QUOTE]
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.308 / Berger VLD's of differing weights
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