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Rifles, Reloading, Optics, Equipment
Reloading
Lighter bullets in 308
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<blockquote data-quote="CONatureBoy" data-source="post: 2214932" data-attributes="member: 118769"><p>The idea that you might "spin a bullet too fast to stabilize it" is an old myth. Faster spin improves gyroscopic stability (up to the point that you spin the bullet to pieces, which is far less common now than in days of yore). If you want to dip your toe into the math, read pp. 157-166 (the first part of Chapter 10: "Bullet Stability") of <a href="https://www.amazon.com/Applied-Ballistics-Long-Range-Shooting/dp/0990920615" target="_blank">Bryan Litz, <em>Applied Ballistics for Long-Range Shooting</em>, 3rd ed. (2015)</a>. He concludes that "different levels of stability [do] not affect . . . drag in any significant way," and the "lift drift" of a "highly stabilized bullet" is "practically irrelevant," changing the height of impact by about 1/2 inch at 1,200 yards in the example he documents in the book. A bullet that exits the muzzle with a gyroscopic stability factor (SG) of 1.5 may end up with an SG of 5.0 at the point of impact, for a long-range shot, because (as the author explains at length) bullets become more stable in flight. Dynamic stability only becomes uncertain when a bullet travels far enough to reach transonic speeds (around 1,300 fps). Litz concludes, "Don't worry if SG is a little higher than 1.5, but don't choose rifling twist rates or bullets whose combination produces an SG below 1.5 in the conditions you expect to shoot in" (pp. 170-171). The real risk is not having enough twist--not having too much.</p><p></p><p>The main undesirable effect of over-spinning a bullet is that it magnifies dispersion due to bullet imperfections. To avoid this effect, Litz recommends (1) using high-quality bullets and (2) using a bullet-twist combination that produces a muzzle SG of under 2.0. So there's your rule of thumb: try for an SG of 1.5-2.0, under the conditions (altitude, temperature, etc.) you plan to shoot in. </p><p></p><p>I'd be very surprised if a novice shooter shot well enough to notice overstabilization effects from a hunting rifle. The math says it's improbable.</p></blockquote><p></p>
[QUOTE="CONatureBoy, post: 2214932, member: 118769"] The idea that you might "spin a bullet too fast to stabilize it" is an old myth. Faster spin improves gyroscopic stability (up to the point that you spin the bullet to pieces, which is far less common now than in days of yore). If you want to dip your toe into the math, read pp. 157-166 (the first part of Chapter 10: "Bullet Stability") of [URL='https://www.amazon.com/Applied-Ballistics-Long-Range-Shooting/dp/0990920615']Bryan Litz, [I]Applied Ballistics for Long-Range Shooting[/I], 3rd ed. (2015)[/URL]. He concludes that "different levels of stability [do] not affect . . . drag in any significant way," and the "lift drift" of a "highly stabilized bullet" is "practically irrelevant," changing the height of impact by about 1/2 inch at 1,200 yards in the example he documents in the book. A bullet that exits the muzzle with a gyroscopic stability factor (SG) of 1.5 may end up with an SG of 5.0 at the point of impact, for a long-range shot, because (as the author explains at length) bullets become more stable in flight. Dynamic stability only becomes uncertain when a bullet travels far enough to reach transonic speeds (around 1,300 fps). Litz concludes, "Don't worry if SG is a little higher than 1.5, but don't choose rifling twist rates or bullets whose combination produces an SG below 1.5 in the conditions you expect to shoot in" (pp. 170-171). The real risk is not having enough twist--not having too much. The main undesirable effect of over-spinning a bullet is that it magnifies dispersion due to bullet imperfections. To avoid this effect, Litz recommends (1) using high-quality bullets and (2) using a bullet-twist combination that produces a muzzle SG of under 2.0. So there's your rule of thumb: try for an SG of 1.5-2.0, under the conditions (altitude, temperature, etc.) you plan to shoot in. I'd be very surprised if a novice shooter shot well enough to notice overstabilization effects from a hunting rifle. The math says it's improbable. [/QUOTE]
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