# ballistic coefficient on bullets

#### Michael Eichele

##### Well-Known Member
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Look at the 100 yard br crowd again with the 14" twist they run for optimum accuracy. If overstabilization didn't exist or showed up more further downrange, why don't they run 8" twists so they could shoot all manner of bullet weights?

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Very good point.

#### goodgrouper

##### Well-Known Member
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and y'all got them smoking

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Sorry!

#### Michael Eichele

##### Well-Known Member
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I believe and the impacts of all the groups and the evidence overstabilization were illustrated in the chapter.

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You are correct.

You may have missunderstood us, I meant they didnt have any mathematical formulas that helps you recalculate BC based on how much you overspin the bullet.

#### Mikecr

##### Well-Known Member
Bullets used in PB BR are completely different than long range bullets, where overstabilization might come into play. They can get away with lower Sg and there are benefits in reduced twist for them. Afterall, there are penalties(in accuracy) for excess twist due to imperfections in bullets.

But this has nothing to do with BC loss that might occur much FURTHER downrange -due to 'Overstabilization'.

I don't have the Sierra book. But given that Sierra went through the effort to demonstrate BC loss due to bullet noses not following trajectory, surely they would try to define it, right? When does it happen? Why did it happen here? What bullets might do it? What RPM &amp; drop speed? When can anyone expect it to happen? What can be done about it?
And here is a formula which defines what we observed, and our drag curves show....A Sierra formula for Overtwist...

This is what I suspect would seperate science from heresay.
Anybody can fire bullets at increased twist rate, and see that BC as well as accuracy suffers beyond some point. But does it differentiate 'Overstabilization' w/regard to trajectory down range?
Sure there are bullets that are known to be dynamically unstable. But not many. And I suspect that you'd have to go way off the beaten path to have an issue with overstabilization. Maybe I'm wrong. Maybe every ballistic program in existance needs to account for this.

I'm getting an itch to setup a radar system, but lack the range -unfortunately.

#### goodgrouper

##### Well-Known Member
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Afterall, there are penalties(in accuracy) for excess twist due to imperfections in bullets.

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I agree with that. But there must be something else to it because custom short range br bullets are almost devoid of imperfections (as close as any projectile can currently be) and yet they still shoot them in 14" twist instead of 9" twists. And, I have fired many super short bullets(55 grain class) through my competitive accuracy 6br 14" twist and have not gotten as good of accuracy as running the 60-70 grain bullets. I would be willing to wager that if my barrel had a 16" twist, they would shoot much better.

And another thing, those little 55 grain bullets can go 3900 fps out of a 6br with a 14" twist, and they do not blow up in route and in fact are far from blowing up and they still don't shoot well. So, they hold together and yet they can't seem to settle down. Why?

I can say that this is a fact:
All bullets exhibit yaw and/or nutation as soon as they leave any barrel regardless of twist rate. It is only a matter of time before they all go "to sleep" in the proper twist /ubbthreads/images/graemlins/shocked.gif. Short bullets usually go to sleep quicker than long bullets, but both their bc's are not as high before this point as they will be after this point-UNLESS the nose of the bullet stays pointed away from the arc of trajectory which is exactly what happens downrange when bullets are shot from too fast of twists. In this case, the bc can go down, then up, then down again downrange. High speed cameras can see this as well as doppler radar can detect it.

#### Mountainsheep

##### Well-Known Member
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Afterall, there are penalties(in accuracy) for excess twist due to imperfections in bullets.

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But there must be something else to it because custom short range br bullets are almost devoid of imperfections

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All bullets exhibit yaw and/or nutation as soon as they leave any barrel regardless of twist rate. It is only a matter of time before they all go "to sleep" in the proper twist . Short bullets usually go to sleep quicker than long bullets

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I hope I’m not out of line for jumping in here, but it seems you guys are describing unexplained physical reactions and movements of the projectiles in flight, once they have left the confines of the rifle barrel. In theory, any projectile that is propelled by a force must release energy as it meets any resistance; if this were true then (and I’m speaking on a microscopic scale) the bullet itself would actually physically expand and contract once the barrel no longer restricts its outside surface. In theory the bullets surface would raise and fall away from its leading edge, over its entire length, giving the impression of a wave moving across its surface. I think that prior to the bullet striking its terminal destination, its greatest reaction would be from its initial contact with resistance as it leaves the barrel; a mini impact, if you will. The bullet surface would the act similar to a surface of a pond when you toss in a stone; the first waves of the ripple are greater and more pronounced, eventually calming down to a more even and smoother flow, resulting in less aerodynamic disruption (both forward and laterally through rotation). This would explain why shorter bullets stabilize, or “go to sleep” over less distance.

As far as the mathematical theories to calculate these actions, you might research quantum mechanics and be able to apply some of the derivatives of Rheology, which is the study of the deformation and flow of matter under the influence of an applied stress.

This is a great thread, very interesting and educational. I feel discussions on topics like this are very good for our sport and prove that LRH is not an irresponsible act, as our detractors would have others believe.

Thanks,
Dave

#### Michael Eichele

##### Well-Known Member
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This is a great thread, very interesting and educational. I feel discussions on topics like this are very good for our sport and prove that LRH is not an irresponsible act, as our detractors would have others believe.

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Well put Dave. Some, who dont understand might say we have way too much time on our hands. I say when it comes to cleanly harvesting the game we hunt and the time we need to ensure that those once in a lifetime tags we get dont gp unfilled, you cant put enough time in to this!