Posted: 9/4/2005 8:15:21 AM EDT
I just started read all the information I can find on twist rate and barrel length.
I would like to start a thread on this subject with anyone that can help us put this in simple terms. My questions are: What is the optimum twist ratio for bullet weight? Is there a chart showing the ratio of twist to barrel length? Example: If you have a 24” barrel with 1 twist per eight inches, then the revolutions the bullet makes out the barrel is three. So will this formula work for a table to show the best. Barrel Length / Twist Rate = Ratio 24” / 8 = 3 18” / 9 = 2 16.5/ 7 = 2. 357 Please keep it simple and help us whom failed calculus comprehend the subject. 





You need to match the bullet to the barrel twist. The bullet you use needs to meet the needs of the application and be stablized by your barrel. I don't know if a simple table would be sufficient. There are too many bullets and too many applications to make a generic table. I would guess that you want something more than the generic manufacturer suggestions.
Barrel twist is a function of bullet length. But since bullet wt. is more universally recognized, the wt. is used instead. Heavier bullets are longer than lighter bullets of the same caliber. To calculate the twist needed to stabilize a specific bullet, the Greenhill formula comes in handy to give you a close approximation. No calculus required. Just a calculator and a micrometer. The Greenhill Formula calculates the twist rates of bullets. The formula is as follows: (if bullet velocity is less than 2800 ft/sec) 150 x diameter x diameter  = required spin length of bullet (if bullet velocity is greater than 2800 ft/sec) 180 x diameter x diameter  = required spin length of bullet Hopes this helps. 


Here is George Greenhill's formula for calculating the correct rate of twist for firearms, His formula is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers. This can be simplified to:
Twist = 150 X D2/L Where: D = bullet diameter in inches L= bullet length in inches 150 = a constant This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula. This will give you a useful approximation to the desired twist. This formula was based on a bullet with a specific gravity of 10.9, which is about right for the jacketed lead core bullet. Notice that bullet weight does not directly enter into the equation. For a given caliber, the heavier the bullet the longer the bullet will be. So bullet weight affects bullet length and bullet length is used in the formula. I'm not a mathematician so please don't ask me to explain this formula in detail 

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A small but important "addendum".....
Bullet length refers to the "bearing surface" length... that is, the "flat" portion of the body of the bullet that will be in contact with the barrel surface. That portion of the bullet that curves toward the point and is not in contact with the barrel surface serves only to "split" the air, and contributes to the ballistic coefficient that is published for most common bullets. If you really get into this, David Tubb makes a "bearing surface" comparator. Tubb's bearing surface comparator 


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