# Barrel twist rates



## WhutHurtz (Jan 2, 2008)

I have what seems to be a very basic question. In general in any given caliber heavier bullets will be longer than lighter bullets. Why do the longer bullets need to have faster spin (barrel twist) rates to be stabilized?


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## darkgael (Feb 10, 2006)

I cannot give you a complete answer but...I believe that the need for faster twist rates is a result of the fact that bullets do not rotate perfectly around their centerlines but "yaw" or oscillate, much like a football does when it is thrown. The longer the bullet, the greater the amount of oscillation there will be at the tip. Increased rotational speed stabilizes this oscillation, again, much like what happens to a football when thrown fast as opposed to slowly.
Eventually, as the bullet rotation slows, the bullet will destabilize, yaw will increase, becoming more and more severe and then the bullet will tumble.
Bullets fired too slowly out of a particular barrel - even "fast twist" - will not stabilize properly because lower velocity will not provide enough rotational speed.
Anyway....I believe that that is substantially correct. Others may have better info.
Pete


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## Csquared (Sep 5, 2006)

Pete, a bullet doesn't spin perfectly, or "oscillates" as you called it, because of imperfections in the bullet. If the bullet's perfect, it will spin perfectly...eventually. If it is NOT perfect, which no bullet is, no amount of rotational speed will correct it. In fact, more rotational speed will make it WORSE! That's why benchrest shooters use twist rates only as fast as absolutely necessary to stabilize the bullet they intend to use in competition.

You basically explained why ANY bullet needs to spin, but the question is VERY deep and requires an answer just as deep...and I'm anxiously awaiting an answer!


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## WhutHurtz (Jan 2, 2008)

Hi Pete.
I understand the item about twist rate and bullet speed. I read something about Winchester testing their 1st generation of FailSafe hunting ammo at reduced muzzle velocity so they could get impact speeds at a reduced distance. (IE: 300yd impact speeds at 100yd test distance.) The field results didn't compare with test results, the reduced M.V. also impacted the rotational speed of the bullet. It turned out that inertial rotation doesn't degrade as much as aerodynamic drag reduces f.p.s.

CSquared.
When I posted the question I thought that it was something that was well understood by experianced reloaders and shooters. The facts are well established by direct observation, since the advent of the rifled barrel. I'm hoping that someone will be able provide some information as to the why of it.


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## Csquared (Sep 5, 2006)

We understand it, but explaining why is something else! You asked what appears to be a simple question, but there will be nothing simple about the complete answer!

I know that putting piece of pie in the microwave results in warm pie.....but I could never attempt to explain why!


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## southdakbearfan (Oct 11, 2004)

The longer the bullet the farther forward the center of aerodynamic pressure is of the bullets of center of gravity, causing it to need more RPM's to maintain stability in the air. Faster twist = more RPM's = the ability to stabilize longer bullets.


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## Csquared (Sep 5, 2006)

...but why does increased rotational speed negate the problem with aerodynamic pressure you've mentioned?

And what is aerodynamic pressure?


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## southdakbearfan (Oct 11, 2004)

Aerodynamic pressure is basically the forces of the air resistance against the bullet. The faster the twist, the higher the rpms the bullet turns increasing the centrifical force which counter acts the aerodynamic forces and keeps the bullet from wobbling out of balance.


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## WhutHurtz (Jan 2, 2008)

Hi BearsFan.
I might be confused, but sounds to me that you are describing areodynamic drag. We know that a bullet loses speed because of drag and that there are designs to reduce that (boat tail bullets vs flat base bullets). I'm sure it's related, see the rest of my reply.
I found a discussuion of twist rates and aero drag at the following web page. The math is pretty involved and will require some study, but it looks like it is comprehensive. Scroll down to the section of the page that starts out "*From a series of posts by Steve Faber". I'm sure the answer to my question is in here somewhere!

http://www.fulton-armory.com/ARTwists.html


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## Csquared (Sep 5, 2006)

....see what I mean?????

You've found the info....as soon as you've figured out the short answer please pass it on to me!


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## cwoparson (Aug 23, 2007)

If this doesn't explain or answer the question then it will probable confuse you even more. Read it anyways. http://anarchangel.blogspot.com/2007/01 ... ology.html


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## Csquared (Sep 5, 2006)

GREAT info, CWO. That's the most "reader friendly" explanation I've seen yet on a physics issue as complex as that. I think equating aerodynamic force to leverage is what makes your explanation so easy to follow. The bearfan was spot on, but your post filled in the blanks... for me anyway.

Thanks for the info....both of you!

Dan


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## WhutHurtz (Jan 2, 2008)

Hi CWO.
I like the explanation you found a lot better than the one I found. It explained the 'whats and whys' to a degree that satified my curiosity and the reason for my original post.

BearFan,
It looks like you were on the right track with the areodynamics. I don't want to sound like I was blowing you off. I didn't realize the center of preassure/center of gravity relationship was so complex in relation to fps.

Thanks to everyone that posted responses and gave insights.


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## southdakbearfan (Oct 11, 2004)

WhutHurtz said:


> Hi BearsFan.
> I might be confused, but sounds to me that you are describing areodynamic drag. We know that a bullet loses speed because of drag and that there are designs to reduce that (boat tail bullets vs flat base bullets). I'm sure it's related, see the rest of my reply.
> I found a discussuion of twist rates and aero drag at the following web page. The math is pretty involved and will require some study, but it looks like it is comprehensive. Scroll down to the section of the page that starts out "*From a series of posts by Steve Faber". I'm sure the answer to my question is in here somewhere!
> 
> http://www.fulton-armory.com/ARTwists.html


Aero drag and the Aero pressure are the same and different, Aero drag is the resistance of how the air flows around the bullet, just like water through a pipe, less head is lost on two 45 degree pipe angles than one 90 degree angle. Aero pressure as I was referring to is more the summ of the forces on the leading edge, which if the bullet wasn't spinning, would cause it to tumble end over end through the air, and which causes keywhole wobbling if it isn't spinning enough.

Another crazy thing is a flat base bullet stablized better in the air than a boattail, but a boattail has less drag. Anyway, seems like between all of us we have a resonable explanation.


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## MagnumManiac (Feb 19, 2008)

WhutHurtz,
The answer is quite simple,a bullet is given rotation so that it stays pointy end to the target.The reason long bullets need more spin is because of the differentiation from its centroid to its longitudinal axis.Put simply.if you have a short piece of wood,you can hold your fingers closer to the middle to make it balance on your hands,when it is longer you need to move your fingers away from its cetroid to make it balance,if you don't it will fall/yaw away from its centroid.
Hope that makes sense to you.
*MagnumManiac*
:sniper:


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## Csquared (Sep 5, 2006)

Please explain to us Yankees how you can balance a board at some point other than it's center if it's long enough????

Must be a "down under" thing. :wink:


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## southdakbearfan (Oct 11, 2004)

Csquared said:


> Please explain to us Yankees how you can balance a board at some point other than it's center if it's long enough????
> 
> Must be a "down under" thing. :wink:


I think what magnum was trying to say is the rear of the bullet is heavier than the front section, like if you put a weight on that 2 by 4 on one end he is balancing. So the center of the bullet lengthwise, is not the centroid of the weight.


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## DuaneinND (Jan 22, 2005)

Bullet shape and weight is a factor along with the length.
Case in point is the 70gr Speer semi round nose. This bullet was designed for use in a 14 twist 222Rem.
BT bullets tend to be less stable for a couple of reasons.
Imperfections in concentricity are multiplied by the gas escaping as the bullet starts to exit the muzzle. As a bullet exits more of the "gas" pressure escapes on one side than the other which would cause any bullet to "tip", this is magnified for a BT because part of the bullet is still inside the barrel and is "pushed" even more off center. 
A less than perfect crown will multiply the effect.
Most of the time a BT bullet has a shorter bearing surface and has a higher o'give which requires a slightly faster twist, which once again multiplies any imperfections in the bullet or crown.
Then there are all the things mentioned in the earlier posts.


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## MagnumManiac (Feb 19, 2008)

Csquared said:


> Please explain to us Yankees how you can balance a board at some point other than it's center if it's long enough????
> 
> Must be a "down under" thing. :wink:


Csquared,
Sorry, I made a boo boo.(It was late when I posted).
What I meant to say is that as any object that is bullet shaped needs by necesstiy to have the shank of the bullet lengthened to increase it's weight.It is therefore a drawback by design to need a faster twist to stabilise such long bullets,because the centroid of said bullets is moved FURTHER to the rear of the bullet.This is what I was trying to say in regard to the plank of wood,as you try to move away from its centre point,the more it tries to fall to one end.
A good example is FAT SHORT pistol bullets,they nearly all have slow twists,a fast twist is not needed to stabalise them because there centroid is very nearly perpendicular to their horizontal axis.
*MagnumManiac*
:sniper:


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## Savage260 (Oct 21, 2007)

> BT bullets tend to be less stable for a couple of reasons.
> Imperfections in concentricity are multiplied by the gas escaping as the bullet starts to exit the muzzle. As a bullet exits more of the "gas" pressure escapes on one side than the other which would cause any bullet to "tip", this is magnified for a BT because part of the bullet is still inside the barrel and is "pushed" even more off center.
> A less than perfect crown will multiply the effect.
> Most of the time a BT bullet has a shorter bearing surface and has a higher o'give which requires a slightly faster twist, which once again multiplies any imperfections in the bullet or crown.


Why are almost all of the target bullets, at least the ones I have seen, BT's if they are less stable? I thought the BT helps stability in bullets for longer ranges? Now I am confused.(not out of the ordinary for me)


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## southdakbearfan (Oct 11, 2004)

Target bullets are BT to increase the ballistic coefficient therefore decreasing drop over distance. The boattail lengthens the bullet, therefore putting the centroid of mass further out of whack with the centroid of the length of the bullet, therefore needing more spin to stabilize. Therefore I have a headache reading this whole post again, lol.


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