Why not 1/8 twist?

[tylerw02]

That goes back to what I was saying above. More than length of the bullet and twist matters in making a bullet stable. The length of bearing surface, the shape, the type of rifling, the velocity, the pressure, the powder charge...it all matters.


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[CC556]

I did a facinating stability experiment not too long ago where I'd have lost the bet if I'd put money on the outcome. Switch to 30 cal mode for a second.... I got some 220 gr soft point bullets with a B.C. that was so poor, you'd not bother shooting them very far at all. So I tried to make subsonics out of them with my 12 twist barrel.

They shot 8 MOA and were yawing when the passed through the paper at 100 yards. For the hell of it, I loaded them to full power, and behold. They were stabile and shot 1 MOA or so. I'd have guess that the increased RPM would be about offset by the increase wind resistance, but not even close. They shot stabile.

Not terribly surprising, figure in a 12 twist barrel at around 1000fps they were doing 60k RPM whereas speeding them up to even 2500fps nearly triples that rotational speed.

[DreadPirateMoyer]

Isn't stability an absolute state. A bullet is either stabile or it isn't. No? There is no "more stabile" or "too stabile"...

Correct. You are either gyroscopically stable or you're not. There's no in between (excluding external factors like wind, bullet construction [for example, a flaw in the copper jacket can be exacerbated as the bullet spins faster], and other things). I believe MistWolf's comment was more in regards to the bullet barely being above that stability threshold, meaning as it slows down, it will hit the non-stable point faster than it would in a faster twist barrel.

As for the bullet being more stable as it slows down, I don't see how that's true. Maybe less affected by external factors, but as far as gyroscopic stability, regardless of if the bullet loses forward velocity faster than rotational velocity, it's still losing rotational velocity, and that's the only variable in the equation with a bullet of static mass/size/etc. A loss in rotational velocity will eventually destabilize the bullet as it crosses between the threshold of gyroscopically stable and not. The forward velocity doesn't affect that.

[markm]

Not terribly surprising, figure in a 12 twist barrel at around 1000fps they were doing 60k RPM whereas speeding them up to even 2500fps nearly triples that rotational speed.

Yep. I did those very calculations at the time of the experiment.

[CC556]

Correct. You are either gyroscopically stable or you're not. There's no in between (excluding external factors like wind, bullet construction [for example, a flaw in the copper jacket can be exacerbated as the bullet spins faster], and other things). I believe MistWolf's comment was more in regards to the bullet barely being above that stability threshold, meaning as it slows down, it will hit the non-stable point faster than it would in a faster twist barrel.

As for the bullet being more stable as it slows down, I don't see how that's true. Maybe less affected by external factors, but as far as gyroscopic stability, regardless of if the bullet loses forward velocity faster than rotational velocity, it's still losing rotational velocity, and that's the only variable in the equation with a bullet of static mass/size/etc. A loss in rotational velocity will eventually destabilize the bullet as it crosses between the threshold of gyroscopically stable and not. The forward velocity doesn't affect that.

Straight from a paper Bryan Litz wrote:

http://www.appliedballisticsllc.com/...eBulletsP2.pdf

One last point regarding stability: Starting as soon as the bullet leaves the barrel, gyroscopic stability begins to increase. The reason is simply because the forward motion of the bullet is decreasing much faster than the rotational speed of the bullet. In other words, de-stabilizing effects are going away quicker than stabilizing effects. The result is that Sg grows as the bullet goes downrange.

[DreadPirateMoyer]

He's talking about external factors and/or using incorrect terminology. Gyroscopic stability has nothing to do with those: http://en.wikipedia.org/wiki/Miller_twist_rule

[tylerw02]

This is true to a point. It's a bell shape curve. I had a problem and couldn't figure it out. My rifle would shoot physically smaller groups at 200 and 300 yards with Berger VLDs.


As in 1" at 300, 1.25" at 100 or something crazy like that....not talking MOA here. I did some searching and red some Litz. He actually has a graphic model of the 7mm 180 VLD demonstrating that the bullet does indeed become more stable in flight. However, it moves from a yaw to stable. Not stabile and continuing to become "increasingly more stable" if I understand it correctly.


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[CC556]

He's talking about external factors and/or using incorrect terminology. Gyroscopic stability has nothing to do with those: http://en.wikipedia.org/wiki/Miller_twist_rule

So it's your position that Bryan Litz doesn't know what he's talking about?

[markm]

This is true to a point. It's a bell shape curve. I had a problem and couldn't figure it out. My rifle would shoot physically smaller groups at 200 and 300 yards with Berger VLDs.


As in 1" at 300, 1.25" at 100 or something crazy like that....not talking MOA here. I did some searching and red some Litz. He actually has a graphic model of the 7mm 180 VLD demonstrating that the bullet does indeed become more stable in flight. However, it moves from a yaw to stable. Not stabile and continuing to become "increasingly more stable" if I understand it correctly.

Yeah... that whole VLD "bullet going to sleep" thing is facinating.

[DreadPirateMoyer]

It's my position that math and science don't change because an expert says so, and gyroscopic stability only changes with regards to rotational velocity with all other factors being equal.