Barrel Twist Question?
- Thread starter Ducimus
- Start date
If the twist is not quick enough, the bullet does not have enough rpm's to keep it from tumbling end over end... = not accurate.
Compare a football being kicked to a nice long spiral pass....
Bullets need enough twist to stabilize. Slightly more twist than needed is better than not enough twist... too much twist may cause a bullet to spin so fast it comes apart in flight.
Long bullets need quicker twists...
A bullet needs to be stable for the best accuracy.
Factory rifles do not give you an option, they select what they feel is the best compromise for most bullets that will be used.
Here is one article... http://www.riflebarrels.com/articles/bullets_ballastics/bullet_imbalance_twist.htm
Compare a football being kicked to a nice long spiral pass....
Bullets need enough twist to stabilize. Slightly more twist than needed is better than not enough twist... too much twist may cause a bullet to spin so fast it comes apart in flight.
Long bullets need quicker twists...
A bullet needs to be stable for the best accuracy.
Factory rifles do not give you an option, they select what they feel is the best compromise for most bullets that will be used.
Here is one article... http://www.riflebarrels.com/articles/bullets_ballastics/bullet_imbalance_twist.htm
Long Range Canuck
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guntech said:
Excellent comparison, mind if I borrow that one sometime guntech?
Pure Energy
CGN Ultra frequent flyer
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- A river runs through it.
Max Owner said:
Yes, a 1:7 twist will stabilize a heavier bullet better than a 1:12 twist.
I found this information in "The ABC's of Reloading, 7th Edition" (2004 KP Books), I am sure that more information on it could be found by searching for the "Greenhill Formula."
The formula gives a close approximation of the ideal twist for any given bullet length (given the same bore diameter, heavier bullets are longer than lighter bullets).
Twist rate = 150 * D^2/L (150 times bullet diameter squared, divided by bullet length).
So for example, a 55 grain .223 bullet, .647 inches long:
Ideal twist rate = 150 * (0.224)^2 / 0.674 = 11.2, or about 1:11.
Now, lets look at a heavier .223 bullet, 70 grains and .785 inches long:
Ideal twist rate = 150 * (0.224)^2 / 0.785 = 9.6, so a twist of 1:9 or 1:10 would be best for this bullet.
*
EDIT*
A quick google of "greenhill formula" turned up this site:
http://www.loadammo.com/Topics/July01.htm
I didn't read it all but I skimmed it and it seems to cover the topic pretty well if anyone is interested in more detail. It brings velocity into the picture. A quick simple way to decide what twist rate you want if you are faced with many choices and you know what type of bullet you want to use.
The formula gives a close approximation of the ideal twist for any given bullet length (given the same bore diameter, heavier bullets are longer than lighter bullets).
Twist rate = 150 * D^2/L (150 times bullet diameter squared, divided by bullet length).
So for example, a 55 grain .223 bullet, .647 inches long:
Ideal twist rate = 150 * (0.224)^2 / 0.674 = 11.2, or about 1:11.
Now, lets look at a heavier .223 bullet, 70 grains and .785 inches long:
Ideal twist rate = 150 * (0.224)^2 / 0.785 = 9.6, so a twist of 1:9 or 1:10 would be best for this bullet.
*
EDIT*
A quick google of "greenhill formula" turned up this site:
http://www.loadammo.com/Topics/July01.htm
I didn't read it all but I skimmed it and it seems to cover the topic pretty well if anyone is interested in more detail. It brings velocity into the picture. A quick simple way to decide what twist rate you want if you are faced with many choices and you know what type of bullet you want to use.
Last edited:
Max Owner said:
It is more like a higher rate of twist is required to stabilize a longer bullet, not just a heavier bullet. The shape and where the mass is center are important.
The best thing anyone can do, if they are going to rebarrel a rifle and are concerned about twist, is to decide on what bullets you wish to use in what cartridge and tell your gunsmith or barrel supplier that info. They will decide on the best twist. Usually you can not have a barrel that will shoot the lightest (shortest) bullet and the heaviest (longest) bullet equally well. Only in larger diameter calibers you can do that.
Look at this chart from Shilen: http://www.shilen.com/calibersAndTwists.html
Bullet length as opposed to weight (although for a given caliber and bullets of similar material - heavier means longer) is the issue re: twist. A child's "top" can be spun and stays "asleep" at fairly slow rpm's cause it is short and fat ...however if you tried to spin a pencil on it's lead point...you'd need a lot more "rev's" to put it to sleep
