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Posted: 12/14/2025 7:17:14 PM EDT
[Last Edit: HobbitHunter][Edited]
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The gun community has mostly boiled down bullet stability to simply 1:x twist rate = good to go for this xx grain - xx grain range of bullet weights. The reason is because it's become so frustrating to try and explain why to people that just don't want to know. You should want to know, because it's not whole picture. The real issue is will this twist rate stabilize this specific ammunition. It's a combination of twist rate, bullet specs, physical bullet construction, velocity, and environmnetal factors that determine the bullet's stability. So, instead of looking online at a twist rate chart, you should instead be looking at an online bullet gyroscopic stability calculator and a bullet stability chart. Here's a good bullet stability calculator: https://jbmballistics.com/cgi-bin/jbmstab-5.1.cgi Here's a quick reference bullet stability chart: 0.0 -> 1.0 gs (gyroscopic stability) = unstable 1.1 -> 1.4 gs = marginally stable 1.5 -> 2.9 gs = stable 3.0 -> infinity = instability introduced from bullet itself Gyroscopic stability is not bullet revolutions per minute. And here's a good site explaining all this more in depth from both a renowned bullet manufacturer as well as a (if not the most) renowned ballistician, bryan litz. https://bergerbullets.com/shoot-better/shooting-knowledge/what-do-the-results-of-the-twist-rate-calculator-mean/ Here's the man himself, bryan litz, explaing it in a short for those that somehow made it to this point, but still ain't got time fo dat readin  Spinning Bullets and Gyroscopic Stability Hope this helps. Edit 1, fixed short Edit 2, fixed title and added gyroscopic stability is not rpm. |
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Originally Posted By Mister_H: The calculator doesn’t account for barrel length. Twist rate is calculated by complete revolutions per inch, so barrel length should be factored when determining the appropriate twist rate. ??? First, twist rate is normally inches per revolution, i.e., one turn in 10 inches, 1 in 10 inches, or 1/10. The twist and the muzzle velocity will give you the spin of the projectile in RPM. Twist (REV/inches) X 12 inches/foot X Muzzle Velocity (feet/second) x 60 seconds/minute = rotational speed in RPM The barrel length is immaterial, and if the twist is progressive, only the twist rate at the muzzle is considered. For a given loading, changing the barrel length will change the muzzle velocity, but then changing the cartridge will also change the muzzle velocity. All we are concerned with in the velocity at the muzzle, therefore the cartridge and barrel length are immaterial. |
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The other thing to know is that marginal stability does not necessarily mean poor grouping, but it does mean compromised BC. I've shot bullets at a SG ~ 1.2 that shot bughole groups on target. Never in 1K years would I have believed that a .30 cal 13 twist would stabilize a 175gr Berger VLD, but it did. |
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20 years ago I called Hornady, the receptionist transferred me to a product engineer. Can I shoot 162gr A-Max in a 1:12 barrel? How fast do I have to push it? IIRC, the answer was along the lines of 3000fps would be good, 2700-2800 might work if I was lucky. Fortunately I had a 26" barrel to get it up to speed. |
I think the hardest thing for good LE working for good agencies to really absorb is that there are whole departments full of exactly the complete fuckheads we rail against here. - vectorsc
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Originally Posted By HobbitHunter: The gun community has mostly boiled down bullet stability to simply 1:x twist rate = good to go for this xx grain - xx grain range of bullet weights. The reason is because it's become so frustrating to try and explain why to people that just don't want to know. You should want to know, because it's not whole picture. The real issue is will this twist rate stabilize this specific ammunition. It's a combination of twist rate, bullet specs, physical bullet construction, velocity, and environmnetal factors that determine the bullet's stability. So, instead of looking online at a twist rate chart, you should instead be looking at an online bullet gyroscopic stability calculator and a bullet stability chart. Here's a good bullet stability calculator: https://jbmballistics.com/cgi-bin/jbmstab-5.1.cgi Here's a quick reference bullet stability chart: 0.0 -> 1.0 gs (gyroscopic stability) = unstable 1.1 -> 1.4 gs = marginally stable 1.5 -> 2.9 gs = stable 3.0 -> infinity = instability introduced from bullet itself Gyroscopic stability is not bullet revolutions per minute. And here's a good site explaining all this more in depth from both a renowned bullet manufacturer as well as a (if not the most) renowned ballistician, bryan litz. https://bergerbullets.com/shoot-better/shooting-knowledge/what-do-the-results-of-the-twist-rate-calculator-mean/ Here's the man himself, bryan litz, explaing it in a short for those that somehow made it to this point, but still ain't got time fo dat readin  https://www.youtube.com/watch?v=Oz0Umw9n3bgHope this helps. Edit 1, fixed short Edit 2, fixed title and added gyroscopic stability is not rpm. There are other, larger, misconceptions about bullet stability. Many people believe that if as soon as the stability factor drops below 1.0, the bullet will tumble immediately after leaving the barrel. This is not the case. An "unstable" bullet is just one whose yaw increases with time rather than decreases, how fast is quite variable. Some "unstable" bullets will punch reasonably round holes in paper out to a few dozen yards, however, dispersion will be larger than a stable bullet. The other misconception is "if this bullet is stable in this twist, it will always be stable . . ." NO. Temperature has a lot to do with it, not only does muzzle velocity drop with temperature, the air density increases. Denser air will increase the overturning moment. And, it does not have to be "arctic temperatures". Let us look at an example, the infamous M193 fired in a 1-14 twist barrel: The bullet fired at a muzzle velocity of 3300 fps in a 1-14 twist on a 95° F day had a stability factor of 1.2, and generally could hold a pattern of just over 1/2 a MIL. At 25° F the stability factor had dropped to about 1.0, and the dispersion had grown to about 5/8 MIL. At 0° F, the stability factor is just under 1.0, about 0.95 with a 1 MIL dispersion. Just for comparison, with a 1-12 twist you get the following: . . . . . . . . . . . . Stability Temp . . . . . . . Factor . . . . . . . . Dispersion 95° F . . . . . . . . 1.5 . . . . . . . . . . . . . . . 0.4 25° F . . . . . . . . 1.3 . . . . . . . . . . . . . . . 0.4 0° F . . . . . . . . . 1.2 . . . . . . . . . . . . . . . 0.4 And, the idea that all bullets of a given diameter, and the same length will have the same stability factor for a give twist. Stability "calculators" like the JBM one linked in the OP, are not true calculators, but approximators. The gyroscopic force required to keep a bullet stable depends on the location of the center of pressure, or the location where all of the aerodynamic forces average out to be, relative to the location of the center of gravity. These stability approximators assume a bullet with the density of a copper jacketed lead core bullet, so the length to weight ratio assumes a nose profile that is achievable with such a bullet. All-copper, and solid lead bullets bullets have differing densities and may have a considerably different aerodynamic properties. Which takes us to the last thing, "Marginally Stable". Bullets are either stable, or they are not stable. It is a binary property, there really is no such thing as "marginally stable". However, we have seen that there are many factors that reduce the stability factor, temperature, velocity, air density, inaccurate modeling of the aerodynamics of the bullet, etc. For this reason a sloppiness factor was required for the "calculators" to account for the fact that it might spit out a stability factor greater than 1.0, but in the real world the bullet would sometimes be in the unstable region. Now, all of this is STATIC STABILITY, only. There is a whole other world of DYNAMIC STABLITY that effects a bullets flight, and the fact that you can have bullets that are statically stable, but dynamically unstable. |
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Originally Posted By lysanderxiii: Which takes us to the last thing, "Marginally Stable". Bullets are either stable, or they are not stable. It is a binary property, there really is no such thing as "marginally stable". True. What is your recommended word for discussing bullets that have a compromised flight characteristics until the coning motions damp out? |
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Originally Posted By Mister_H: The calculator doesn’t account for barrel length. Twist rate is calculated by complete revolutions per inch, so barrel length should be factored when determining the appropriate twist rate. Once the spin is imparted it keeps spinning. I don’t think extra barrel contact does much to the RPMs beyond more barrel is more burn time if the powder is selected to use the extra length. Not sure if it’s in the Litz video or I saw it else where. The bullet spin rpm doesn’t appreciably slow while it’s on its flight, forward velocity of course does. An 18” 1-10 twist and a 26” 1-10 twist are going to have the same RPMs if the MV is equal. |
The only hyphenated names I like are cartridge names......30-06, 30-40, 38-55 etc.
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As above, barrel length by itself is immaterial since it is velocity and twist rate which matter for minimum bullet stability. What many fail to take into account is excessive bullet stability. If overstabilized the bullet may keep its orientation the same as it had when it left the barrel, which is detrimental to the effective BC. An adequately stabilized bullet will maintain allignment with the bullet's trajectory, while an overstabilized bullet may not follow the trajectory, maintaining an effective nose high orientation. The result is a bullet which exposes too much of its profile to the air, causing excess drag and a reduced effective BC. Not a big deal for most shooters, but it can have a negative affect at very long ranges, affecting wind drift and trajectory. . |
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Originally Posted By lysanderxiii: There are other, larger, misconceptions about bullet stability. Many people believe that if as soon as the stability factor drops below 1.0, the bullet will tumble immediately after leaving the barrel. This is not the case. An "unstable" bullet is just one whose yaw increases with time rather than decreases, how fast is quite variable. Some "unstable" bullets will punch reasonably round holes in paper out to a few dozen yards, however, dispersion will be larger than a stable bullet. The other misconception is "if this bullet is stable in this twist, it will always be stable . . ." NO. Temperature has a lot to do with it, not only does muzzle velocity drop with temperature, the air density increases. Denser air will increase the overturning moment. And, it does not have to be "arctic temperatures". Let us look at an example, the infamous M193 fired in a 1-14 twist barrel: The bullet fired at a muzzle velocity of 3300 fps in a 1-14 twist on a 95° F day had a stability factor of 1.2, and generally could hold a pattern of just over 1/2 a MIL. At 25° F the stability factor had dropped to about 1.0, and the dispersion had grown to about 5/8 MIL. At 0° F, the stability factor is just under 1.0, about 0.95 with a 1 MIL dispersion. Just for comparison, with a 1-12 twist you get the following: . . . . . . . . . . . . Stability Temp . . . . . . . Factor . . . . . . . . Dispersion 95° F . . . . . . . . 1.5 . . . . . . . . . . . . . . . 0.4 25° F . . . . . . . . 1.3 . . . . . . . . . . . . . . . 0.4 0° F . . . . . . . . . 1.2 . . . . . . . . . . . . . . . 0.4 And, the idea that all bullets of a given diameter, and the same length will have the same stability factor for a give twist. Stability "calculators" like the JBM one linked in the OP, are not true calculators, but approximators. The gyroscopic force required to keep a bullet stable depends on the location of the center of pressure, or the location where all of the aerodynamic forces average out to be, relative to the location of the center of gravity. These stability approximators assume a bullet with the density of a copper jacketed lead core bullet, so the length to weight ratio assumes a nose profile that is achievable with such a bullet. All-copper, and solid lead bullets bullets have differing densities and may have a considerably different aerodynamic properties. Which takes us to the last thing, "Marginally Stable". Bullets are either stable, or they are not stable. It is a binary property, there really is no such thing as "marginally stable". However, we have seen that there are many factors that reduce the stability factor, temperature, velocity, air density, inaccurate modeling of the aerodynamics of the bullet, etc. For this reason a sloppiness factor was required for the "calculators" to account for the fact that it might spit out a stability factor greater than 1.0, but in the real world the bullet would sometimes be in the unstable region. Now, all of this is STATIC STABILITY, only. There is a whole other world of DYNAMIC STABLITY that effects a bullets flight, and the fact that you can have bullets that are statically stable, but dynamically unstable. I was taught that 1.0 ->1.4gs is the margin of error. Yes, at 1.0 a 1:1 ratio is achieved, the binary lightswitch has been flipped, but there's stlll that margin of error (be it human or otherwise) that is overcome by shooting for 1.5gs minimum. |
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Originally Posted By SteelonSteel: Not sure if it’s in the Litz video or I saw it else where. The bullet spin rpm doesn’t appreciably slow while it’s on its flight, forward velocity of course does. It's the reason the gyroscopic stability increases as the bullet flies downrange. As the velocity decreases, so does the overturning moment on the bullet. Since the rotational velocity decreases substantially less than forward velocity, the gyroscopic stability increases the further downrange the bullet flies. |
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Originally Posted By bpm990d: True. What is your recommended word for discussing bullets that have a compromised flight characteristics until the coning motions damp out? Inaccurate. You can have a bullet that is very stable, but due to extreme yaw at launch, wanders before the yaw damps out. A stability calculator will not tell you anything about this behavior. The M193 bullet can see 10 to 13 degrees of yaw at launch. |
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Originally Posted By lysanderxiii: Inaccurate. You can have a bullet that is very stable, but due to extreme yaw at launch, wanders before the yaw damps out. A stability calculator will not tell you anything about this behavior. The M193 bullet can see 10 to 13 degrees of yaw at launch.  |
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Originally Posted By lysanderxiii: Inaccurate. You can have a bullet that is very stable, but due to extreme yaw at launch, wanders before the yaw damps out. A stability calculator will not tell you anything about this behavior. The M193 bullet can see 10 to 13 degrees of yaw at launch. What do you mean by wanders? You mean a lack of precision? |
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Originally Posted By bpm990d: What do you mean by wanders? You mean a lack of precision? Anything that flies through the air with a yaw produces lift. Lift produces movement in the direction of lift, and a body in motion remains moving in that direction until an opposing force is introduced. If you look at the tip trace shown above, you will see the yaw is not uniform but has a secondary precession that holds the yaw at certain angle a bit longer than others. Depending on the magnitude of the primary and secondary corkscrewing, there can be considerable drifting. EDIT: At 00:06:00 if you watch the projectile, you will see the yaw is not uniform. https://www.youtube.com/watch?v=xpJ8EoGmLuE&t=332s |
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Just one practical example. A 1:9 twist 16 inch barrel can be counted on to stabilize 70 grain jacketed lead core bullets at typical 5.56 velocities. From that barrel five shot groups are just over 1 MOA. Try to shoot a 70 grain Barnes TTSX loaded to the same typical 5.56 velocities from that same barrel. This is a very long for weight monolithic copper bullet. The result: The 70 TTSX is not stable and keyholes badly, spraying a 100 yard target with a five shot group of about 12" and all bullets tumbling wildly, striking the target sideways. The difference is bullet construction and length. A 1:8" 16" barrel stabilizes those bullets nicely. |
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