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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Bullet Spin and Performance
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<blockquote data-quote="Michael Eichele" data-source="post: 248607" data-attributes="member: 1007"><p>Stability factor is a number calculated by formulas to determine the optimum twist for our bullets or optimum bullets for our twist. A factor of under 1.0 is concidered unstable. 1.5 is concidered perfect. Over 1.5 is concidered stable, not neccesarily optimal. Greenhill had a formula but there are much much more accurate formulas used today.</p><p></p><p>Many things go into calculating the stability factor. Some include barrel twist, bullet demensions (including meplat width, nose style and length, ogive radius, bearing surface, base diameter, boat tail length, and overall length, bullet diameter and weight), velocity, air density and a given bullets "specifec gravity". Such as lead, copper jacketed lead, solid lead, ect...Zinc has a very low specifec gravity and tungsten has a very high specifec gravity. The lower the number, the more it takes to stabilize it. This is why solid copper bullets need a bit more twist than do jacketed lead. Jacketed lead has a SG of 10.7 where solid copper is 8.96.</p><p></p><p>Obviously some factors above are more critical than others. The 3 most critical elements for the materials we typically use are twist, length and diameter. All others are factors just not as big as those 3.</p><p></p><p>Calculating them is tricky. This is why most here use ballistic calculators either on a PC or online.</p><p></p><p>I have been lucky enough to have some loads with a 1.5 SF. I believe they have been my best overall performing loads ever. I have had above published BCs, AND they always seem to drift in the wind less than predicted, minimal spin drift and increadiby accurate from 0-1000 yards. I dont really have any "scientifec" proof that these are better than a 1.0 or a 2.0 but the results in my opinion are enough to convince me that 1.5 really is a perfect number. Anybody will have a very hard time convincing me that any other number is better.</p><p></p><p>Clear as mud right?</p></blockquote><p></p>
[QUOTE="Michael Eichele, post: 248607, member: 1007"] Stability factor is a number calculated by formulas to determine the optimum twist for our bullets or optimum bullets for our twist. A factor of under 1.0 is concidered unstable. 1.5 is concidered perfect. Over 1.5 is concidered stable, not neccesarily optimal. Greenhill had a formula but there are much much more accurate formulas used today. Many things go into calculating the stability factor. Some include barrel twist, bullet demensions (including meplat width, nose style and length, ogive radius, bearing surface, base diameter, boat tail length, and overall length, bullet diameter and weight), velocity, air density and a given bullets "specifec gravity". Such as lead, copper jacketed lead, solid lead, ect...Zinc has a very low specifec gravity and tungsten has a very high specifec gravity. The lower the number, the more it takes to stabilize it. This is why solid copper bullets need a bit more twist than do jacketed lead. Jacketed lead has a SG of 10.7 where solid copper is 8.96. Obviously some factors above are more critical than others. The 3 most critical elements for the materials we typically use are twist, length and diameter. All others are factors just not as big as those 3. Calculating them is tricky. This is why most here use ballistic calculators either on a PC or online. I have been lucky enough to have some loads with a 1.5 SF. I believe they have been my best overall performing loads ever. I have had above published BCs, AND they always seem to drift in the wind less than predicted, minimal spin drift and increadiby accurate from 0-1000 yards. I dont really have any "scientifec" proof that these are better than a 1.0 or a 2.0 but the results in my opinion are enough to convince me that 1.5 really is a perfect number. Anybody will have a very hard time convincing me that any other number is better. Clear as mud right? [/QUOTE]
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