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Twist Rate-Stability-Accuracy-Confusion

I was just experimenting with the Berger twist rate calculator and was shocked to see that several of the bullets I am using would be considered "understabilized" or at least "marginally stabilized" based on the twist rates of my rifles. Almost every twist rate chart I had found previously would show a 1:10 twist barrel appropriate for 212 and 220 grain bullets such as an ELD-X, but this calculator says not. Apparently, the old charts are an oversimplification and do not properly account for a bullets length in addition to its weight (length being the more important factor as I now understand). This would indicate they are useless for the new high BC bullets that everyone is moving to. The SG (Specific Gravity) rating delivered by the calculator shows anything under 1.5 as "marginally stable", over 1.5 as "Stable". I'm currently looking at an SG of 1.36. Apparently, you can still get good accuracy under 1.5 but you will actually be achieving a lower BC based on how far below the 1.5 you are, accuracy may also suffer. Below 1 would mean extreme instability and keyholing.

My question is if anyone has been achieving best accuracy with what would be considered a "marginally stabilized" bullet or if you have found "marginal stabilization" to be a serious limiting factor. I am wondering if I should continue load development or just choose a shorter, lower BC bullet and throw in the towel before wasting too much time and money.
 
I cannot answer your question.
But

If you look at what many, if not most PPC shooters are using vs what the calculator is spitting out you will see a discrepancy.

The calculator will tell you you need a 12 twist or faster yet the big dogs are winning big matches with a 13 1/2 or 14 twist. Again PPC and common bullets and velocities.

Last year I asked two well known short range hall of fame shooters the same question on different days at different times. Both said exactly the same thing…… don’t trust the calculator.

CW
 
…… don’t trust the calculator.
This... and nominal twist rate (marked on barrel) vs actual barrel twist. We have a 1/9 nominal twist 223 bolt gun 20" that stabilizes way longer bullets than it should. We've shot 80 gr CC, and 88 ELDs through it with acceptable results.
 
Berger uses the Miller Twist formula which is different than the Greenhill formula. Both formulas are not closed form solutions (exact) and assume certain geometries and give conservative twist for most bullets.

Brian Ritz found in his research that when the gyroscopic stability (SG) decreased below about 1.5 the ballistic coefficients decreased. The marginal stability region is between 1.0 and 1.5. It does not mean the bullets will be unstable but that the BC is reduced.
 
Several years ago, Bryan Litz (Chief Ballistician @ Berger Bullets) suggested that a gyroscopic stability coefficient of 1.4 was sufficient to obtain the full intrinsic (maximum) BC of a given bullet. He later increased that number to 1.5, presumably based on further research/computer modeling. During the time between when an Sg of 1.4 being sufficient and now, when apparently an Sg of 1.5 is necessary to obtain the full intrinsic BC of a bullet, nothing changed with respect to what air is made of, the dimensions of a given bullet, etc. So part of the difference between then and now is based on one's perspective, or a sort of relative mind set given the two different values.

The bottom line is that bullets having an Sg of about 1.1 or less will probably exhibit gyroscopic instability. It may only be slightly oblong holes in the paper, or possibly full blown keyholing if the value goes below 1.0. Bullets with Sgs somewhere in the range of 1.20-1.25 up to 1.35-1.40 may appear to have full gyroscopic stability in that they don't leave oblong holes in the target or keyhole, but they are giving up a certain amount of their intrinsic BC. The question really being asked here is should someone running a bullet with an Sg somewhere in the range of 1.25 to 1.35 (or thereabouts) expect to see strange, anomalous, shocking, unusual, weird, or otherwise abnormal behavior. The answer is obviously, "No". Bullet/barrel twist combos that generate Sgs in that range are not going to show any strong unusual behavior. They should not produce oblong holes in targets. They should not keyhole. They should not suddenly fall from the sky at distances past 700 yards. They might simply behave as thought the BC is a little less than expected. In other words, they might move a tiny little bit farther offline than would be expected in a given wind condition. Frankly, that is something that is NOT easy to accurately quantify even if you're looking for it.

So many people that are running barrel twist rate that is just slightly low for their bullet of choice (i.e. not quite fast enough to generate an Sg of 1.5 or greater and garner the full intrinsic BC of the bullet) will never even know there is a "problem". That is, until they go to Berger's website and use the Twist Rate Calculator, which unfortunately sometimes unintentionally may leave users with impression that bullets having an Sg of less than 1.5 are about to fall out of the sky, thereby creating concern, possibly even panic or hysteria. ;)

If one is going to have a new barrel chambered, of course it would be wise to choose a blank with sufficient twist rate to generate an Sg of 1.5 or greater with the intended bullet(s). However, running bullets with Sgs in the 1.25 to 1.35 range is not a problem. Most will never even know something is [theoretically] wrong with such a setup. If bullets aren't leaving oblong holes in the targets or keholing, just shoot it and have fun.
 
SG (Specific Gravity) rating delivered by the calculator
SG in this context refers to a variable name in the code used for ballistics. It stands for "Stability, Gyroscopic" and so it became SG for short.

Gyroscopic stability can be a matter of luck when you get into that gray zone below 1.5

If you ever played with a spinning top as a kid, you witnessed the difference if the tip was a little off or if the center of gravity was off, then you saw a difference in how the thing wobbled before it petered out.

Long projectiles are subject to engraving on the rifling that may be off, or the construction of the bullet may be off, causing the CG of any given sample to be off the spin axis to some degree.

These little errors, plus any bad luck with unburnt powder exhaust or crown errors, will all play into the margin or dispersion of how they fly once out of the muzzle. Higher quality bullets and barrels let you get closer to SG 1, but for margin the ballisticians "recommend 1.5 or greater". Notice I put that in quotes because these are not calculations of force equilibrium that guarantee anything, they are models that use parameters that have been shown by experimentation to predict stability rather than cause it.

When you keep the SG above 1.5, you just have a better chance that the CG and spin axis won't wobble and cause a "dirty" flight. No guarantee with that since they are just "curve fitting" stability test data rather than predicting forces. No way to know how good or bad your bullets, crown, or rifle really are.

So in the end, this calculation is not one where there is a physics of forces, but they are more of a study of useful parameters that are matched up to an observable behavior. You can get away with less than 1.5 as long as you understand the risks of getting too close to 1 or poor quality bullets, etc..

There are many places in science and engineering like fluid and aerodynamics, heat and mass transfer, etc., where the "math" isn't really calculating force equilibrium that predicts behavior. These fields use parametric studies to try and "manage" a chaotic event. Often, the "rules of thumb" are the result of lots of experimental observation with careful manipulation of the parameters to understand their scale factors and exponents, which is nothing more than fitting the calculated data till it matches the observations.

Gravity in a vacuum is something we can directly calculate, but things like aerodynamic drag and gyroscopic stability are something we just try to manage. YMMV
 
Slowburn,

There has been a number of discussions over the last few years on this very subject. The one I've paid the most attention to is the Berger .20 cal 55 gr Varmint Match bullet. There has been a lot of back and worth about the Berger calculator saying 8-twist or 8 1/2 twist. There is a lot of folks including myself who have shot the 1:9 twist with that bullet for many years in the 20BR and the improved cases in that wildcat. It works fine and has produced 1 1/2" groups at 500 yds with that set-up for me. I've shot it past 700 yds on PD's with success. I've also used a 1:10 twist in the .20-22/250 past a 1000 yds on PD's with the Berger 55 gr bullet. It works and shoots very tight groups. Barrel life is another problem with the .20/250...Lol!

The problem that can happen "with that particular bullet" is if you spin it to fast with a large case behind it...it can come apart.

Like the old saying "the proofs in the pudding"!

Best of luck.
 
Don't forget the role of air density in all this.
I imagine many folks don't realize that their actual stability is higher/lower due to local air densities.
Your 1.39 Sg under ICAO conditions (59degF, 0' above sea level)(std Berger calc air density) rises to 1.5 Sg at 85degF/900' ASL.
 
I have shot both short range benchrest and long range F-class. I am by no means a nationally ranked shooter in either discipline but I am ranked as a high master at long range F-class. My experience tells me that at short range you can easily get away with a Sg lower than 1.4. One thing to also keep in mind is that the Berger twist rate calculator is not accurate for flat based bullets often used at short range. However at long range with heavy high BC bullets I have always done better with a Sg of 1.5 or higher. It’s not that a 1.3 or 1.4 won’t shoot but based on my own experience at mid and long range I have generally experienced tighter groups when the Sg is at least 1.5. When it comes to twist rate for long range I prefer to err on the fast twist side. The other factor as pointed out by “mikecr” above is that air density does come into the equation and I generally shoot at just slightly above sea level in the South Carolina “low country”.
 

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