Twist Rate Formula

Grtwhthntr

Grtwhthntr

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Recently read an article in Jan./Feb. Rifle Shooter magazine regarding the calculation of optimal twist rate.

Here is the formula:

Bullet Length divided by its diameter = X
150(constant) divided by X times diameter(caliber)

Example

.257 Roberts 120 gr spitzer
1.125/.257=4.38
150/4.38=34.25x.257=8.8

So a good twist rate for this rifle rounding up would be 9", which is the normally accepted twist rate.

Now my .204

32gr V-max
.635/.204=3.11
150/3.11=48.23x.204=9.84

So the recommended twist rate rounding up would be 10", but, my SPS 700 Varmint has a 12" twist, so theoretically an even a lighter/smaller bullet would be optimal.

Am I missing something or why would a manufacturer produce a twist rate that factory bullets aren't even manufactured for. My rifle shoots excellent, just crunching the numbers to educate myself a little. Is there some deal at the lighter end of the bullet spectrum that invalidates this formula?
 
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Twist rates are only a guide. There are a number of other factors to consider regarding twist/stabilization - i.e. actual bullet length, velocity, and length of barrel. Just because a twist rate isn't "recommended" doesn't mean that you can't shoot certain bullets - I know of a few 12 twist .224 cal barrels that were able to stabilize 70 grain bullets even though by the book they weren't supposed to. In your example of the .204 I doubt that you want that much twist as you won't be able to stabilize the light bullets too well (too fast = jacket flying apart) a 10 twist may be good for higher weight bullets (i.e. 40 grains or higher but would be probably too fast for the lighter weight bullets which the .204 R is trying to use to go over 4000 FPS (i.e. 32 grain bullets). As well theoretical knowledge doesn't always translate to real world results.
 
I thank you for the response, exactly what I was looking for, presumed there must be other factors involved but just wasn't sure why. Too fast=jacket flying apart makes sense. Fairly new to the technical stuff, but find it fascinating.
 
Grtwhthntr said:
Recently read an article in Jan./Feb. Rifle Shooter magazine regarding the calculation of optimal twist rate.

Here is the formula:

Bullet Length divided by its diameter = X
150(constant) divided by X times diameter(caliber)

Example

.257 Roberts 120 gr spitzer
1.125/.257=4.38
150/4.38=34.25x.257=8.8

So a good twist rate for this rifle rounding up would be 9", which is the normally accepted twist rate.

Now my .204

32gr V-max
.635/.204=3.11
150/3.11=48.23x.204=9.84

So the recommended twist rate rounding up would be 10", but, my SPS 700 Varmint has a 12" twist, so theoretically an even a lighter/smaller bullet would be optimal.

Am I missing something or why would a manufacturer produce a twist rate that factory bullets aren't even manufactured for. My rifle shoots excellent, just crunching the numbers to educate myself a little. Is there some deal at the lighter end of the bullet spectrum that invalidates this formula?
Click to expand...

What stablity factor does this equation for twist produce in the fired bullet?
You haven't said. It should be 1.5 to 2.0. Less than 1.5 in cold dense air, the bullet could be unstable, and greater than 2 is more than necessary.

NormB
 
NormB said:
What stablity factor does this equation for twist produce in the fired bullet?
You haven't said. It should be 1.5 to 2.0. Less than 1.5 in cold dense air, the bullet could be unstable, and greater than 2 is more than necessary.

NormB
Click to expand...
I honestly have no clue, like I said new to the tech stuff, now that you got me interested in this stability factor where can I find out more info?
 
I had heard that the formula you are using has a dividing line of 2800 FPS. For bullets at 2800fps and less, use the 150 factor. For bullets with a velocity over 2800fps, use a factor of 180. With the 180 factor used, the twist rate calculates out to 11.8, so the 1-12 twist is about right....unless you are shooting under 2800fps.
 
The formula is called the Greenhill formula and there is a great deal of information on the formula on the net..

The thing to remember is it is not strictly the "twists" that stabilizes a bullet, it is the RPM induced by the twist. As long as the bullet is able to spin to its optimal RPM, you can theoretically stabilize with any twist, but the problem is the velocity and what exactly is the optimal RPM for your given bullet.

If the bullet manufacturers would tell you what the optimal RPM would be, you could decide on a twist based on a reasonable expectation of velocity.
 
I posted the greenhill formula on this site awhile back. Since that time a new formula has come out that take into account the recent design advantage of these needles that Berger and Lapua and others are producing. The creater was Don Miller and his articles can be found in Precision Shooting magazine.

Here is a great explanation on stablizing bullets and optium twist rate; it references the 7mm but the basis are the same for all calibers.
http://bryanlitz.bravehost.com/articles/7mmNumberOne.pdf

Once you have read over this article you can click the 2nd link and play around with the optimal twist rate for whatever caliber you want using Don Miller's formula.

http://accurateshooter.wordpress.com/2008/06/03/calculating-bullet-rpm-spin-rates-and-stability/
 
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