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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Bullet stability
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<blockquote data-quote="DocUSMCRetired" data-source="post: 1952469" data-attributes="member: 85458"><p>Just to touch on a couple of things in here:</p><p></p><p>1) Our calculator uses Station Pressure (altitude is irrelevant) for the calculation.</p><p></p><p>2) Something to consider. When you think about stability you are somewhat concerned with the overturning moment (end over end). This has a lot to do with the Center of Pressure vs Center of Gravity. So a shorter fatter bullet will be naturally more stable needing less twist. Make a bullet longer, and you increase the distance between the CP and CG which in turns means you need to spin it faster to prevent that overturning moment and yaw due to instability.</p><p></p><p>3) With the above understood, a bullet loses forward velocity much faster than it loses rotational velocity. So it is safe to assume that you have higher stability down range.</p><p></p><p>4) A bullet that isn't stable from the start will not later become stable. So you need the bullet to leave the muzzle with the stability you want for it to transition into subsonic without incident. Bullets stabilize in the first couple procession cycles. So in essence it never gets better if you don't start off stable, an unstable bullet will not clean up at say 200 yards etc.</p><p></p><p>4) A +/- 0.1 error when using the Miller formula is generally accepted.</p><p></p><p>5) Dynamic stability is a whole different beast but we do have the tools to measure it and our system isn't as simple on the inside as we make it look on the outside. Our system even accounts for pressure changes and gravities effects as you shoot uphill/downhill just to explain how complicated it is internally.</p></blockquote><p></p>
[QUOTE="DocUSMCRetired, post: 1952469, member: 85458"] Just to touch on a couple of things in here: 1) Our calculator uses Station Pressure (altitude is irrelevant) for the calculation. 2) Something to consider. When you think about stability you are somewhat concerned with the overturning moment (end over end). This has a lot to do with the Center of Pressure vs Center of Gravity. So a shorter fatter bullet will be naturally more stable needing less twist. Make a bullet longer, and you increase the distance between the CP and CG which in turns means you need to spin it faster to prevent that overturning moment and yaw due to instability. 3) With the above understood, a bullet loses forward velocity much faster than it loses rotational velocity. So it is safe to assume that you have higher stability down range. 4) A bullet that isn't stable from the start will not later become stable. So you need the bullet to leave the muzzle with the stability you want for it to transition into subsonic without incident. Bullets stabilize in the first couple procession cycles. So in essence it never gets better if you don't start off stable, an unstable bullet will not clean up at say 200 yards etc. 4) A +/- 0.1 error when using the Miller formula is generally accepted. 5) Dynamic stability is a whole different beast but we do have the tools to measure it and our system isn't as simple on the inside as we make it look on the outside. Our system even accounts for pressure changes and gravities effects as you shoot uphill/downhill just to explain how complicated it is internally. [/QUOTE]
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