Does velocity and atomspheric temperature have any effect on bullet stability? Specifically when using a marginal twist as 1:9 for 75 and 80 grain .224 bullets.
Range Report Bullet stabilization and Velocity
- Thread starter RingingSteel
- Start date
Re: Bullet stabilization and Velocity
Two types of stability; passive and dynamic.
Passive stability is gyroscopic.
Dynamic stability is aerodynamic.
So, the short answer is 'yes'. The link covers it in much more detail.
In essence, bullets are aerodynamically unstable, i.e. their center of pressure is ahead of their center of mass. This is like putting the feathers on the front of an arrow.
To compensate for this, bullets are spun to generate gyroscopic forces that tend to resist the bullet's weathervaning tail first.
When gyroscopic stability is less than marginal, aerodynamic forces tend to overcome the gyroscopic forces, and bullets tumble/keyhole.
Generally, aerodynamic forces decay faster than gyroscopic forces; so if the bullet achieves gyroscopic stability at all, it will tend to become more stable as the flight progresses.
Using a 1:9" twist at .223-capable muzzle velocities, I've found that instability sets in at around the 75gr bullet weight/length and above. My .223 AR uses a 1:8" twist and can handle heavies, my .223 Varmint M700 uses a 1:12" twist and can't.
When determining twist compatibility, length is the actual determinant factor. Heavier bullets tend to be longer, which is why most folks think in terms of bullet weight (which is actually looking at the issue backward). Bullets made of lighter materials still need a twist that suits their length, and not their weight.
Greg
Two types of stability; passive and dynamic.
Passive stability is gyroscopic.
Dynamic stability is aerodynamic.
So, the short answer is 'yes'. The link covers it in much more detail.
In essence, bullets are aerodynamically unstable, i.e. their center of pressure is ahead of their center of mass. This is like putting the feathers on the front of an arrow.
To compensate for this, bullets are spun to generate gyroscopic forces that tend to resist the bullet's weathervaning tail first.
When gyroscopic stability is less than marginal, aerodynamic forces tend to overcome the gyroscopic forces, and bullets tumble/keyhole.
Generally, aerodynamic forces decay faster than gyroscopic forces; so if the bullet achieves gyroscopic stability at all, it will tend to become more stable as the flight progresses.
Using a 1:9" twist at .223-capable muzzle velocities, I've found that instability sets in at around the 75gr bullet weight/length and above. My .223 AR uses a 1:8" twist and can handle heavies, my .223 Varmint M700 uses a 1:12" twist and can't.
When determining twist compatibility, length is the actual determinant factor. Heavier bullets tend to be longer, which is why most folks think in terms of bullet weight (which is actually looking at the issue backward). Bullets made of lighter materials still need a twist that suits their length, and not their weight.
Greg
Re: Bullet stabilization and Velocity
Greg did well on it, the short answer is
Yes, it has a lot of impact when you change the atmospheric conditions.
The higher the Density Altitude, the more aerodynamically stable things are. In the summer months when DA's are commonly in the 2500-4000' range I used to be able to shoot the 75Amax from my 9tw. When the DA was in the 1200-1500' Range I'd get 1-1.5MOA response that just "never seemed to shoot" and when it was cold, I could barely hit the burm at 500yd.
With the 75 BTHP that's very similar in shape but 0.1" shorter because of the lack of ballistic tip, they worked all year round down to -1200' DA on friggin cold ass days in January.
Greg did well on it, the short answer is
Yes, it has a lot of impact when you change the atmospheric conditions.
The higher the Density Altitude, the more aerodynamically stable things are. In the summer months when DA's are commonly in the 2500-4000' range I used to be able to shoot the 75Amax from my 9tw. When the DA was in the 1200-1500' Range I'd get 1-1.5MOA response that just "never seemed to shoot" and when it was cold, I could barely hit the burm at 500yd.
With the 75 BTHP that's very similar in shape but 0.1" shorter because of the lack of ballistic tip, they worked all year round down to -1200' DA on friggin cold ass days in January.
Re: Bullet stabilization and Velocity
That's what I found. The HDY 75gr BTHP would shoot in the 24", 1:9" for me, the HDY 75gr A-Max usually wouldn't.
That's what I found. The HDY 75gr BTHP would shoot in the 24", 1:9" for me, the HDY 75gr A-Max usually wouldn't.
Re: Bullet stabilization and Velocity
The effect of velocity on spin stabilization is relatively small compared to other factors as discussed above. For example, the stability factor for 2500 fps MV compared to the std. MV used in these calculations (2800 fps) is decreased by less than 4% because the corrective equation is an inverse cube function [(actual MV/2800 fps) to the 1/3 power]; see Corrective Equations in the link below. So increasing MV can buy you a very small amount of stability, which might make the difference if your twist rate/projectile combination was right on the edge, but otherwise the effect of increasing MV on stability is overshadowed by other effects.
http://en.wikipedia.org/wiki/Miller_twist_rule
The effect of velocity on spin stabilization is relatively small compared to other factors as discussed above. For example, the stability factor for 2500 fps MV compared to the std. MV used in these calculations (2800 fps) is decreased by less than 4% because the corrective equation is an inverse cube function [(actual MV/2800 fps) to the 1/3 power]; see Corrective Equations in the link below. So increasing MV can buy you a very small amount of stability, which might make the difference if your twist rate/projectile combination was right on the edge, but otherwise the effect of increasing MV on stability is overshadowed by other effects.
http://en.wikipedia.org/wiki/Miller_twist_rule
Re: Bullet stabilization and Velocity
The reason I ask is Hornady's manual recommends a minimum of 1:10 twist for a 80gr Amax at 22-250 velocities. That being said I know that most shooting the 80 grain pills from a .223 utilize a 1:8 or faster twist tube. Does the 200fps gain from a 22-250 make a difference in minimun twist rates?
The reason I ask is Hornady's manual recommends a minimum of 1:10 twist for a 80gr Amax at 22-250 velocities. That being said I know that most shooting the 80 grain pills from a .223 utilize a 1:8 or faster twist tube. Does the 200fps gain from a 22-250 make a difference in minimun twist rates?
Re: Bullet stabilization and Velocity
The short answer is that increased MV <span style="font-style: italic">might</span> make a difference if the twist rate is marginal for that projectile. Usually the gains from increased MV aren't huge, but could make the difference in stability if the specific rifle/projectile combo were on the ragged edge of acceptable stability. I once had a rifle that fell into this category and learned a lot from playing around with different loads to see what worked and what didn't.
If you're really interested in this topic, I would suggest getting a copy of Bryan Litz' book, "Applied Ballistics for Long-Range Shooting". There is a chapter specifically about projectile stability that explains the many aspects of what you're asking far more simply and concisely than I can. This book also includes more useful information on other topics than can easily be imagined and is worth every penny. Highly recommended for all LR shooters.
http://www.appliedballisticsllc.com/index_files/Book.htm
If you know the specific projectile dimensions, you can also go to JBM Ballistics and use the following calculators, to see how some these factors change with respect to varying MV:
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi
The short answer is that increased MV <span style="font-style: italic">might</span> make a difference if the twist rate is marginal for that projectile. Usually the gains from increased MV aren't huge, but could make the difference in stability if the specific rifle/projectile combo were on the ragged edge of acceptable stability. I once had a rifle that fell into this category and learned a lot from playing around with different loads to see what worked and what didn't.
If you're really interested in this topic, I would suggest getting a copy of Bryan Litz' book, "Applied Ballistics for Long-Range Shooting". There is a chapter specifically about projectile stability that explains the many aspects of what you're asking far more simply and concisely than I can. This book also includes more useful information on other topics than can easily be imagined and is worth every penny. Highly recommended for all LR shooters.
http://www.appliedballisticsllc.com/index_files/Book.htm
If you know the specific projectile dimensions, you can also go to JBM Ballistics and use the following calculators, to see how some these factors change with respect to varying MV:
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi
Re: Bullet stabilization and Velocity
One addition: Bullet stability is dependent on atmospheric conditions. The atmosphere is 20% more viscous at 0dF than at 100dF. So you can shoot things in the summertime that are unstable in the wintertime. You can also shoot thinigs at altitude that you can't shoot at sea level.
One addition: Bullet stability is dependent on atmospheric conditions. The atmosphere is 20% more viscous at 0dF than at 100dF. So you can shoot things in the summertime that are unstable in the wintertime. You can also shoot thinigs at altitude that you can't shoot at sea level.
Re: Bullet stabilization and Velocity
Wow, that was informative! The greatest thing about this part of the forum is that I have answers to questions have not really needed to ask yet! Have been debating litz's book, will have to get now.
Wow, that was informative! The greatest thing about this part of the forum is that I have answers to questions have not really needed to ask yet! Have been debating litz's book, will have to get now.
Re: Bullet stabilization and Velocity
Bryan's book is really an invaluable resource at many different levels, and all for the price of a couple boxes of factory ammo. You'll be glad you bought a copy!
Bryan's book is really an invaluable resource at many different levels, and all for the price of a couple boxes of factory ammo. You'll be glad you bought a copy!
Re: Bullet stabilization and Velocity
Speaking of stability, can someone answer me this.
I keep hearing that a bullet "can" go to "sleep" or stabilizes at a greater distance than at close range.
I can't wrap my head around the thought that if at 100 yards a particular bullet will group poorly (say 1+ MOA) but magically tighten up at 300 yards. If off at 100 wouldn't that always put it off more at a further distance??
Someone educate me please
Speaking of stability, can someone answer me this.
I keep hearing that a bullet "can" go to "sleep" or stabilizes at a greater distance than at close range.
I can't wrap my head around the thought that if at 100 yards a particular bullet will group poorly (say 1+ MOA) but magically tighten up at 300 yards. If off at 100 wouldn't that always put it off more at a further distance??
Someone educate me please
Re: Bullet stabilization and Velocity
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RTK</div><div class="ubbcode-body">Speaking of stability, can someone answer me this.
I keep hearing that a bullet "can" go to "sleep" or stabilizes at a greater distance than at close range.
I can't wrap my head around the thought that if at 100 yards a particular bullet will group poorly (say 1+ MOA) but magically tighten up at 300 yards.
</div></div>
Due to the yawing and pitching motion the projectile's center of mass describes a spiral (epicyclic swerve). When yawing and pitching damp out (the bullet "goes to sleep") the swerve amplitude or the radius of the spiral decreases. However, the initial radius is usually much smaller than a caliber, so the effect is not responsible for a tighten up of groups at further distances.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RTK</div><div class="ubbcode-body">
If off at 100 wouldn't that always put it off more at a further distance??
</div></div>
I'd say yes. I don't believe there are any effects that are leading to smaller groups at further distances (beside statistics).
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RTK</div><div class="ubbcode-body">Speaking of stability, can someone answer me this.
I keep hearing that a bullet "can" go to "sleep" or stabilizes at a greater distance than at close range.
I can't wrap my head around the thought that if at 100 yards a particular bullet will group poorly (say 1+ MOA) but magically tighten up at 300 yards.
</div></div>
Due to the yawing and pitching motion the projectile's center of mass describes a spiral (epicyclic swerve). When yawing and pitching damp out (the bullet "goes to sleep") the swerve amplitude or the radius of the spiral decreases. However, the initial radius is usually much smaller than a caliber, so the effect is not responsible for a tighten up of groups at further distances.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RTK</div><div class="ubbcode-body">
If off at 100 wouldn't that always put it off more at a further distance??
</div></div>
I'd say yes. I don't believe there are any effects that are leading to smaller groups at further distances (beside statistics).
Re: Bullet stabilization and Velocity
Thanks tob, I didn't think the bullet would decrease it's MOA grouping with distance.
Thanks tob, I didn't think the bullet would decrease it's MOA grouping with distance.
Similar threads
