Twist rate and bullet weight

[stanford]

I could swear that I had posted this question, maybe I didn't hit the save button.

I have been trying to understand the barrel twist rate and the bullet weight to make the perfect combination. I have read quite a few articles, I have tried the calculators but this is very tricky. Can someone help me understand in a nutshell how to come up with the bullet weight that is just right for your gun according to the twist rate of the barrel?

Thanks

[Outpost75]

This plug-in is quick and easy:

http://www.bergerbullets.com/twist-rate-calculator/

Another link using same method

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

A simplified narrative explanation:

http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/
The gyroscopic stability condition

A spin-stabilized projectile is said to be gyroscopically stable, if, in the presence of a yaw angle d, it responds to an external wind force F1 with the general motion of nutation and precession. In this case the longitudinal axis of the bullet moves into a direction perpendicular to the direction of the wind force.
It can be shown by a mathematical treatment that this condition is fulfilled, if the gyroscopic stability factor sg exceeds unity. This demand is called the gyroscopic stability condition. A bullet can be made gyroscopically stable by sufficiently spinning it (by increasing w!).

As the spin rate w decreases more slowly than the velocity vw, the gyroscopic stability factor sg, at least close to the muzzle, continuously increases. An practical example is shown in a figure Go to figure.
Thus, if a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight. The quantity sg also depends on the air density r and this is the reason, why special attention has to be paid to guarantee gyroscopic stability at extreme cold weather conditions.

Bullet and gun designers usually prefer sg > 1.2...1.5, but it is also possible to introduce too much stabilization. This is called over-stabilization.

The gyroscopic (also called static) stability factor depends on only one aerodynamic coefficient (the overturning moment coefficient derivative cMa) and thus is much easier to determine than the dynamic stability factor. This may be the reason, why some ballistic publications only consider static stability if it comes to stability considerations.

However, the gyroscopic stability condition only is a necessary condition to guarantee a stable flight, but is by no means sufficient. Two other conditions - the conditions of dynamic stability and the tractability condition must be fulfilled.

The classical treatment for those less inclined to take shortcuts:

http://www.nennstiel-ruprecht.de/bullfly/gyrocond.htm

The simple answer, too much is better than too little.

[scottfire1957]

Above, a sticky or two:


http://castboolits.gunloads.com/showthread.php?245302-RPM-Threshold-A-Tale-of-Three-Twists-Chapter-II

[scottfire1957]

Part one, also above instickies:

http://castboolits.gunloads.com/showthread.php?208186-RPM-Threshold-barrel-twist-velocity-chart

[Mauser48]

It also depends a lot on bullet length not just weight.

[Larry Gibson]

Unless you are using some really exotic alloy the length of the bullet for a given caliber is the determining factor not the weight of the bullet given a specific velocity.

Larry Gibson

[scottfire1957]

It also depends a lot on bullet length not just weight.


Yeppers, weight and length go hand in hand (within calibers), hence my refrence to the stickies above.

[stanford]

Thanks guys I do appreciate this. I am going to have to dive into this and try my best to understand, I had no idea this got more and more complex.

[leadman]

The gun manufacturers generally use a standard twist rate that will stabilize the popular bullet weights for the cartridge. This is a good guide if you are planning a build.
The 300 Whisper was designed to shoot up to a 240gr jacketed bullet at subsonic velocities. This is why the "standard" twist for this cartridge is an 8 twist. Manufacturers like Thompson Center use the 10 twist expecting more lighter than heavy for caliber bullets will be shot. Then there are the wide assortment of twist for the AR.
Berger also lists the twist rate needed to stabilize some of their VLD bullets which also helps guide me in purchasing their bullets.
Velocity also has an effect on the stability of the bullet.
This is about all I understand about twist rate but found it has served me well. From what I have read there is also varying opinions on what is correct.
Good luck in your quest for knowledge.

[Digital Dan]

Unless you are using some really exotic alloy the length of the bullet for a given caliber is the determining factor not the weight of the bullet given a specific velocity.

Larry Gibson

I admit to having met exotic dancers, but never such styled alloys.

Lads, like Larry said, it is length, not weight. There really isn't any way around it. Where adherents to weight theory get tripped up is the influence of form. Given equal weight and caliber, a VLD bullet form requires faster twist for stability than a round nose given the same velocity. The reason is found in the difference in location on the long axis of center of gravity and center of aerodynamic pressure.

[stanford]

The only issues I have really had with the calculators online is when they require the length of the bullet. I started looking at random bullets online and using random weights, there was never any length listed in the description on any of the boxes.

[tsubaki]

Nosler is pretty good about listing some of their bullets length and suggested twist rates.

[tsubaki]

By the way, I didn't see where you mentioned a specific cartridge/firearm you are studying on.

[Larry Gibson]

The only issues I have really had with the calculators online is when they require the length of the bullet. I started looking at random bullets online and using random weights, there was never any length listed in the description on any of the boxes.

That can indeed be a problem if you don't have the bullets on hand. However, a guess in results in only a guess out with any calculator (garbage in = garbage out). If we want a correct answer we need to input correct data.

Larry Gibson

[Digital Dan]

Might be enough shooting loonies here that a fella could ask, "Hey, how long is a... (pick your bullet)????", and someone here could whip out his calipers faster than Buffalo Bill could draw an inside straight.

Just a thought.

[John Boy]

A simple calculator - Charlie Dell's Twist Formula
http://tmtpages.com/twistrate.htm

More advanced Dell Calculator ... https://xa.yimg.com/df/CB-BOOK/2.6.1.2+C.+DELL%27S+TWIST+FORMULA+WORKBOOK.xls?tok en=ch_OYHXd-j2r-Y8pLydOMaAh5qNhdVMtLgCjGCDbt8RHaZMQSgOPibouBR9GfiC i8z0SdDI0hlZZCXaz5LA4HBrODOpdWNmy2DfTZ59IYK1zSSnRo szf-tYbLzRiZxITJ7HIUv-EEggg6IS6Ggolg0A2bmstxIy33_VrSXdYSoWuczvY11GlKAEGt XdhEHV9xXznq4oCEbH-&type=download