Help me understand rate of twist


March 25, 2013, 05:21 PM
First, I know what 1:9 or 1:7 means, but I want to understand why its important, especially with bullet weights. For example I've read that a 1:7 twist in an AR is more desirable for a heavier bullet than 1:9.

1. General explanation or anything I'm wrong on.
2. Is the extra twist purely for extra speed with a heavier round?
3. What is considered too heavy a round for a lower rate of twist in an AR example? With 55 grain being normal, is 62 grain OK or pushing it? 75 grain?

Thanks in advance.

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Vern Humphrey
March 25, 2013, 05:56 PM
Generally speaking a longer bullet requires a faster twist. You can have a heavy, blunt-nosed bullet that will shoot well with a slow twist, and a lighter, long-nosed, boat tailed bullet that will not shoot well at that twist.

One often used formula is Twist = 150 X (bullet diameter in inches)^ X Sqrt 10.9. This is called the Greenhill forumla.

For higher velocities (above about 2800 fps) substitute 180 for 150.

For other than common lead core bullets, substitute the Specific Gravity of the bullet for 10.9.

March 25, 2013, 06:00 PM
Try to think of it this way.The bullet is like a top spinning on the table. The shorter and wider the top is, the slower it can spin before it tips over. if the top is longer and narrower, it has to spin much faster to stay stabilized. finding out the hard way, I discovered that a 1-12 on a tikka t3 is too slow to stabilize a 68gn in 223. your not hurting anything it just key holes on the paper. As far as what rate of twist you need for which bullet, I can't help you. Heavier the bullet, the longer it is, the more twist you need. Are you looking at buying something? and if you are,what are you guna do with it? Hunt prairie dogs? Coyotes? Long range? or just plink?

March 25, 2013, 07:24 PM
I'm newer as well but I understand that heavier bullets require more stabilization. And it is a ratio- 1:12 means 1 complete revolution in 12 inches. So of course, the lower the second number (1:7 the second number being being the 7) the tighter the turn, the more turns per barrel length = more stabilization. Now the heavier wider analogy is newer still to me. Anyone else agree?

March 25, 2013, 07:33 PM
The actual formula for how much twist is based on length. Because most bullets are made with lead cores and gilding metal jackets you can normally use weight as a shorthand for length.

The problem is weird bullets like tracers. Very long but relatively light weight (trace compound is way lighter than lead) bullets required the 1:7 twist rate to stabilize in 556 NATO.


March 25, 2013, 08:19 PM
The classic Greenhill equation is

T' = 150 / L'

where the twist and the bullet length are in calibers. Removing bullet diameter from twist and length gives the equation often found:

T = 150 * D^2 / L

The Greenhill equation includes no term for muzzle velocity, and several sources suggest replacing the 150 with 180 for muzzle velocities over 2800 fps. Increasing muzzle velocity increases bullet spin, and spin provides the stability. An article in the 11/2001 Single Shot Exchange cites an article by Les Bowman in the 1962 Gun Digest offering an equation which includes muzzle velocity (in fps):

T = 3.5 * V^0.5 * D^2 / L

At 2800 fps, this equation is equivalent to using 185 in the Greenhill equation, and at 1840 fps, this equation is the same as Greenhill's.

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