What are the tell tell signs that your bullets are not stabilizing? Is it possible that a bullet that is not stable at 100 yards will stabilize at a longer distance?
Is it possible to over stabilize a bullet? If a person were to install a 1:8" twist on their 22 or 6MM rifle in order to shoot heavy bullets would they still be abe to use 40gr 22's or 58gr 6mil's.
How much of a factor is velocity in the decision for the rate of twist to install or is the length of the bullet the only consideration? For instance, would a 223Rem. need the same twist as a 223WSSM for the same weight, type, & brand of bullet?

It is possible to have too much spin. As spin increases, any effects due to slight imbalance in the bullet become worse.

I don't agree denton, the bullet making guys say no such thing as overstabilizing. You can spin a thin jacketed bullet too fast & make it come apart, but I haven't seen a problem shooting say 85gr/6.5 out of my 1-8 .260ai. It puts them into tiny little groups all day. better too fast than too slow.

quote:

Originally posted by fredj338:
You can spin a thin jacketed bullet too fast & make it come apart.

I have seen 40 gr hollow points out of a 22-250 come apart and it looked like you shot at the target with a shotgun. Problem was resolved by slowing em down about 250-300 fps.

quote:

Originally posted by hvy barrel:How much of a factor is velocity in the decision for the rate of twist to install or is the length of the bullet the only consideration? For instance, would a 223Rem. need the same twist as a 223WSSM for the same weight, type, & brand of bullet?

I use 2800 FPS as the differentiating velocity. Those below require one twist, those above require another.

Denton is correct. The faster you spin a bullet, the more any imperfections present in the bullets are emphasized.

Todays target and varmint bullets are so very good that these effects are minor, and no one outside competitive benchresters would find them. But, bullets are not perfect, and a lot of hunting bullets are built with terminal performance as the first criteria. Thicker jackets, etc, do not make for "perfectly balanced bullets", and I suspect that even the average shooter with a match grade setup can find the difference between proper twist and "excess twist". In that limited case: hunting bullets and very accurate rifles.

A second issue presents itself at very long range. An "overstabilized" bullet will not nose over as easily, which results in increased drag and reduced velocity. FWIW, Dutch.

hvy barrel,
Here's the link to an thread from a couple of weeks ago about twist rate. It may help.

https://forums.accuratereloading.com/eve/forums/a/tpc/f/2511043/m/240101582

We use gyroscopic stability to overcome aerodynamic overturning moments that result from having the CP(center of aerodynamic pressure) forward of CG(center of gravity). The greater the distance between the two, the greater requirement for increased angular momentum to overcome increased moments. In simple speak, longer bullets require faster twist.

Bullets generally exhibit two modes of gyroscopic precession, fast and slow cycle, one induced by imbalance about the axis of the bullet, the other by additional factors not pertinent to the question of the post. These two processions combine to form an epicyclic precession which, if viewed as a 2 dimensional pattern based on the movement of the bullets nose will resemble a rosette pattern.

The magnitude and evolution of slow cycle precession is a function of Sg or Gyroscopic Stability Factor. If a bullet has a low Sg the magnitude of it precession will slowly increase as the bullet proceeds downrange, much as a top's wobble will increase as RPM decays. It does so for different reasons but the result is comparable. Conversely, if the Sg is high, the slow cycle precession will reduce in magnitude although it will never completely disappear.

The fast cycle precession is caused by imbalance on the bullet's axis and the fact that it transitions from rotating on the central axis as it passes down the bore, to a new rotational axis on the axial CG once clear of the bore. It generally nulls within 150-200 yards, leaving only the slow cycle precession in residence. This is what people are speaking of when they refer to bullets "going to sleep".

To deal directly with the posted question, a bullet must be sufficiently stabilized to have any accuracy at all, that part is well understood I think. The effects of overstabilization are generally only seen by very long range shooters, and are manifested by the effects of yaw of repose and tractibility issues. Side effects of this are that high RPM will cause structural failure in bullets generally considered to have thin jackets and/or soft alloy cores. Too, faster twists cause more deformation of the bullet as it engages the lands, and this has a deliterious effect on accuracy albeit small in most cases.

Low Sg exibits less likelyhood of destruction of the bullet, or deformation of the bullet in comparison to the bullets stabilized with fast twists, but at short ranges they can be measurably more accurate. BR shooters using the 6mm PPC tend to favor this approach using twist rates in the 1:14 range, sometimes slower depending on atmospheric conditions. For a given bullet the overturning moments increase as the bullets begins to decelerates toward the Mach 1.3 range, and thus will likely fall below an Sg adequate to stabilize the bullet.

Indications of instability are diminished accuracy in the best case, or keyhole impact in the worst of circumstances. Early ballisticians measured the oblate nature of holes in paper to evaluate stability, perhaps you can discern this on your targets, likely not. In any case, it is advisable to have a little too much twist than too little. Other factors influence the ability of a given twist rate to stabilize a bullet, such as bore smoothness. Greenhill is an excellent calculating tool but do not rely on it in entirety to determine a minimum twist for a specific bullet. The pros generally use an Sg of 1.3 as a minimum, and that is fairly low as stability factors go, Sg of 1.0 being theoretical unity.

Velocity increases will add some stability for a given twist and barrel but the proper way to deal with the matter is to alter twist rates. So, yes, you may use a lessor twist in the WSSM in comparison to the .223, but it is a fools errand IMO. The .223 bullet may well retain its stability far beyond the WSSM in terms of range. Comparing the benefits of twist rate changes versus velocity is a simple mathematical calculation. Remember that the stability is induced thru angular momentum, not forward velocity. Crunch the numbers and compare RPMs at different velocities and the same twist. Then change the twist rate at the same velocity.

Short answer? Shoot 'em and see what happens. If there are any of the polycarbonate tip bullets out there prone to disentigration due to high vel/fast twist issues I'm not aware of it. Of course those with fast twist guns may not shoot many of them as they are likely interested in long range work, not the forte of the bullets you mention. I've heard a number of folks discuss shooting bullets of the weights you mention as velocities above 4400 fps without problems.

Oh well, I've rambled enough.

Thank you very much for your great contribution, Dan.
It's a comprehensive description of the rules of top and bullet behavior.

Volker

Digital Dan,

I think you have "unmasked" yourself a bit. I will interpret your normally folksy answers a bit differently from now on.

Don't do that now, I was just practicin' my deadpan ...

"CP forward of CG", "the greater requirement for increased angular momentum to overcome increased moments."

"two modes of gyroscopic precession", "the movement of the bullets nose will resemble a rosette pattern"

"depending on atmospheric conditions"

"deliterious effect on accuracy albeit small", "the oblate nature", "use an Sg of 1.3 as a minimum"
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Shoot 'em and see what happens.
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Once you get a Chimera Hunter wound up, he can impress a person!