Intuitively, one would think that if a barrel's twist rate was on the edge of being too slow, that running the projo to the highest velocity possible would help compensate by increasing the spin/time ratio. However, during a range discussion with a fellow shooter, I was told that velocity does not matter, that the bullet will still spin the same number of times in the distance from the muzzle to the target. I am not contending this, as the projo will still only spin say, 1 time every 12 inches. However, if stability is a factor of gyroscopic force, then it would be a factor of revolutions per time, not revolutions per distance, correct? I have always deduced that a heavy-for-caliber projo will do better at higher velocities (in general) because the maximum revolutions per given period of time would be achieved, thereby increasing the gyroscopic effect of the bullet and by default stabilization. Am I on the wrong track? This guy seemed pretty certain the velocity would not matter, and that twist rate could not be overcome, but I am having a hard time accepting this.
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Range Report Can velocity compensate for twist rate?
- Thread starter ahhshoot
- Start date
Velocity improves stability, all else being equal, but not very much for small changes in velocity.
You're correct that the time rate of spin (RPM's) stiffens the spin axis, but there's an offsetting effect of velocity as well.
As velocity increases, the aerodynamic drag force acting on the bullets' center of pressure also increases, and this has a de-stabilizing effect.
As it turns out, the stabilizing effect of increasing spin rate is 'stronger' than the destabilizing aerodynamic effect, so in the end, stability improves slightly for higher velocity.
You can use the Berger online Ballistics calculator to see how much velocity affects stability: http://www.bergerbullets.com/twist-rate-calculator/
Take care,
-Bryan
You're correct that the time rate of spin (RPM's) stiffens the spin axis, but there's an offsetting effect of velocity as well.
As velocity increases, the aerodynamic drag force acting on the bullets' center of pressure also increases, and this has a de-stabilizing effect.
As it turns out, the stabilizing effect of increasing spin rate is 'stronger' than the destabilizing aerodynamic effect, so in the end, stability improves slightly for higher velocity.
You can use the Berger online Ballistics calculator to see how much velocity affects stability: http://www.bergerbullets.com/twist-rate-calculator/
Take care,
-Bryan
Thank you Bryan,
I'm going to print out your response and tape it to his bench this weekend. That should settle it! haha. Excellent explanation, in truth I did not account for or consider the aerodynamics being altered by the increase in velocity but that makes sense as well. Thanks for the link and explanation.
The above answer was very good, but left room to get more quantitative.
The dependence of stability on velocity goes as the 1/3 power of velocity. This means a 10% increase in muzzle velocity only gives you a 3.2% increase in stability.
The dependence of stability on barrel twist rate goes as the square of twist rate. This means a 10% increase in twist rate increases stability by 21%.
No he didnt, he is talking about changing rate of twist. Like spinning a magic knob on your barrel that would tune it from 1:9 to 1:8.1. I doubt anyone disagrees that the best way to affect stability is rate of twist. However, once you've already got gun in hand, that option is out the window. Short of changing the barrel of course.
I always thought this was the case with the 22-250 caliber. High velocity heavy bullet in a slow twist ?
If you run the numbers, you'll find that most 50-55gr bullets in 14 twist barrels don't have an excessively high stability factor even in 22-250. Their relatively short range job description usually never reveals this, so their owners are none the wiser.
Even in my 22-243win with 52SMK at 4100fps, I only show a stability factor of 1.23. You need about 1.4 or higher before things become good and stable. Dropping to a 12 twist does pretty good, but if you do the RPM calculations, you could go to a 10 without creating any issues for yourself even at 22-243 velocities. With proper considerations given, I think that most fast 22 cal's are under-spun from the factory.
Open tipped match bullets often have higher stability than indicated by the most widely used stability formula. This formula assumes a bullet of constant density, and the open space at the front does not meet this assumption. We have developed and tested modifications of the original Miller stability formula for plastic tipped and open tipped match bullets.
It turns out that 52-53 grain OTM bullets are very stable in 1 in 14" twist rates. I don't have the dimensions of the 52 SMK, but the 52 grain Berger has an SG of 1.59 from a 1 in 14" twist at 2800 fps under standard conditions (sea level, 59 deg F). Boosting the velocity to 4100 fps increases the SG to 1.81.
You can find a link to the spreadsheet here:
Stability Formula for Aluminum Tipped Bullets (Spreadsheet attached)
Interesting. That's quite a different number than that provided by the stability calculator in most ballistic apps and the berger stability calculator.
I'd be interested in listening to you elaborate on this more if you wouldn't mind.
I've heard arguments that support your position in the past, but was always of the thought that I'd rather error on the side of caution when determining stability.
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Your response resonated a less than desirable tone. After reading that paper, I did a little research. There were parts of that paper that weren't tracking with my experiences. That research turned up some very interesting things about you.
Ever hear the saying "If you can't explain something simply, you don't understand it well enough?"
I won't make the mistake of attempting to engage you in serious discussion again.
Ever hear the saying "If you can't explain something simply, you don't understand it well enough?"
I won't make the mistake of attempting to engage you in serious discussion again.
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We are always eager to consider hard data that suggests our stability formulas may not be as accurate as our data suggests (5%).
I'm sorry if reference to a published paper describing the details struck a bad chord with you. I had already provided the main reason why the original Miller stability formula underestimates stability for OTM bullets: the assumption of constant density is not fulfilled due to the empty space in the tip.
This was a very simple explanation, so I referenced the full paper when more details were requested.
One can achieve fine results using the overly conservative original Miller twist rule implemented by JBM, ColdBore, Berger, etc. But Don knew his formula can be improved for plastic tipped and open tipped match bullets. We worked together to improve the formula for plastic tipped bullets and co-authored two Precision Shooting articles on the development and validation of the stability formula for plastic tipped bullets.
We had begun work on the OTM formula when Don passed away, so that work was completed with other colleagues.
The advantage of using more accurate stability formulas is that one realizes that one can often shoot higher BC bullets or other longer bullets under certain conditions. At higher altitudes, there is no problem shooting 69 grain OTM bullets from a 1 in 12" twist or the higher 60-62 grain OTM or plastic tipped bullets from a 1 in 14" twist. The extra BC can really come in handy for long shots at prairie dogs on the high plains.
The tipped TSX bullets are a great choice for deer and antelope with .224 bullets. The plastic tipped stability formula will tell you which ones you can shoot under given environmental conditions with a given twist rate.
The user of the term "overly conservative" would suggest to error on the side of caution regarding twist rate is a bad idea. Point in fact, after reading other posts of yours from this forum, and several others... your interactions are laced with hostility in nearly every instance, either directly or indirectly.
Your findings may be correct on paper, but I believe them to be overly optimistic from a real-world shooters perspective. It's one thing to call out stability at the muzzle, but another thing entirely when considering distances. Case in point, most people believe a 9 twist 7mm to perform adequately with berger 180's, 175SMK's, or the new 175ABLR. Some even advocate a 10 twist. For closer ranges, that may just be correct. However, real world shooting indicates that as you approach 1500yds and beyond, those bullets lack the stability to remain precisely predictable. An 8 twist however, has proven to give us some very predictable results. We've seen similar results at distance with a 338 and going to a 9.3 twist as opposed to the more standard spun bores. In the above situations, as well as others, the so-called "conservative" model you cite, was found to be too liberal when considering real shooting situations as the firing solution was pushed beyond normal target engagement distances.
At this point I'll simply say that I will be joining a long and ever lengthening line of people that apparently do not agree with you. After reading your interactions with some people that I call friends, and others that I have read for years and respect... there is no way I will be able to respond to your posts objectively in the future. Your posting reeks of the elitist fodder that stifles productive conversation by trying to artificially inflate a topic rather than address it simply and concisely. Conversely, those you are found attacking routinely on this forum and others have made it their life's pursuit to explain the most complex ideas in the simplest manner. After reading your crass remarks everywhere else, I have little doubt of how you'll receive this post, so I have no expectation of an adjustment in your behavior. Yet even you can't be so blind as to think there aren't consequences for the way you've been conducting yourself.
You are confusing the ideas of gyroscopic stability and dynamic stability. The gyroscopic stability is the smallest when the bullet leaves the muzzle.
However, depending on the bullet design, a bullet can have excellent gyroscopic stability at the muzzle and then develop dynamic instability downrange.
The original post was asking a question about gyroscopic stability. The discussion of dynamic stability is much more complicated, so I choose to keep the discussion within the boundaries of the original question.
Increasing the twist rate may mitigate the problem for some bullets with dynamic stability issues. It will not help with others. Increasing gyroscopic stability in hopes of increasing dynamic stability is not an approach that will work reliably.
Consider for example the 168 and 190 grain SMKs (in .308). The 168 has dynamic stability issues in cases where the 190 does not, even though (other factors being the same), the gyroscopic stability of the 168 SMK is higher. (And one need not depend on stability formulas to know the gyroscopic stability of the 168 SMK is higher. The SG of both of these bullets was measured by Bob McCoy at the BRL spark range.)
This was what sparked the discussion initially. My friend and I are working up loads for our relatives/friends' hunting rifles. They are just hunting guns and the guys who own them aren't interested in changing barrels or throwing a lot of money at the rifles, they just want to get the optimal load for their hunting rifles. So changing a barrel is not really an option, at least at this point. What we have found is that we have very skinny, short barrels with lots of freebore and slow twist rates. One of the rifles, a 7mm mag, has had the barrel cut down to 18". The owner of this rifle said he had accuracy issues and he was shooting 150 grain bullets, we feel that this is because the short barrel isn't pushing the 150 to adequate velocities to properly stabilize, due to the fact that the twist rate on this particular rifle was probably suited for 150 grain bullets with the original 26" tube. So we are hoping to resolve this issue by using 110gr bullets and pushing them as fast as possible. None of the shots these guys take on deer are beyond 400 yards and they are usually within 100, so a heavy projo with a great bc is of no real benefit. However, a good grouping rifle is.
Haven't tried anything short of the 150 gr factory ammo so far. I have some gameking 140's that fly very well out of my 26" 7mag, so I might try some of those. But, I really don't see a reason for him to shoot anything bigger than 110's anyway. The deer aren't that big down in Georgia where he is hunting, and the shots are almost always within 200 yards. I think for the savings in recoil and trajectory over the practical distances fired, we might jump straight to the 110's. His wife also uses the rifle sometimes, and she might appreciate the recoil with the lighter bullets as well.
I played with that calculator and it doesn't seem to agree with the Berger reloading manual. For the 7mm 168 grain the manual says you should use a 1 in 10 twist, but the calculator says 1 in 8.5. I can't make the calculator recommend 1 in 10.
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I'm not confusing anything. Although this is a predictable response from you that tracks with the other responses I've seen. If someone disagrees, they obviously must be confused, right?
A "stability" problem that shows up at 1500 yards is a dynamic stability problem.
Gyroscopic stability problems will show up inside of 50 yards. If the bullet does not outright tumble, then it will demonstrate increased drag and large shot to shot variations in drag if there is a gyroscopic stability problem.
When one calls a paper on gyroscopic stability into question based on observations from long range shots, they are confounding the issues.
You said your observations did not track with our paper on gyroscopic stability of OTM bullets.
I asked for hard data regarding gyroscopic stability and you referred to observations at 1500 yards.
Here's a link with brief descriptions:
Bullet Stability
The attached graph from ARL helps illuminate the difference. The flight characteristics and damping of pitch and yaw in the first 100 yards indicate good gyroscopic stability. The behavior in region 3 shows a dynamic stability problem that will impact the bullet flight at long range.
Attachments
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Stability can be a very difficult thing to nail down and see consistency across the board, especially when looking at 'recommended' twist rates. All recommended twist rates are based on multiple assumptions including:
What is considered an adequate stability factor? Usually 1.2 to 1.5 is considered minimal, and this can have a big impact on the 'recommended' twist.
Also, what conditions do you calculate stability for; normal conditions or worse case scenario?
These and other things combine to result in a 'range' of recommended twist rates.
Historically, we (Berger) have recommended twist rates that get you an SG of 1.4 or higher in normal conditions. However, recent testing has shown that BC may be compromised slightly for anything less than 1.5. That's why the Berger online calculator shows somewhat marginal stability for a 1:10" twist rate on the 7mm 168 VLD. I am a bit surprised that it's showing you 1:8.5" is required for that bullet, that seems excessive for normal conditions. If you could tell me what inputs resulted in the calculator suggesting 1:8.5", that would help.
Rest assured that the original twist recommendation will 'work' for the bullet, meaning it will be accurate, and effective, etc. There just may be the possibility that you're leaving a few % of BC on the table if the bullet isn't getting full stability.
Take care,
-Bryan
I was mistaken. I was changing between 180 grain hunting vld, and 168 grains to see the difference. I got 8.75 for the 180 grains, and 9.25 for 168. I was using 2750 velocity, 70F, 375ft. If anybody is wondering. The suggested value only shows if the twist rate is to slow. I also don't have my manual in front of me so I don't know if 2750 good velocity to use. I'll check it later.
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Bryan, responses like those above is why you'll continue to have my support. Thank you for your contributions. I only wish you had more time to participate on this board, as your insights are very appreciated.
I really like reading this discussion, lots of good information here that I will have study more. I found this thread looking to see if it was possible to find a way to improve accuracy of lighter bullets (55gr) through a faster twist rate barrel (16" 1:8) The barrel seems to really like 73 gr bullets but they aren't as economical as 55gr bullets.
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