Disclaimer:  I am a student, not an expert, and these are my class notes.  If you need an expert, consult any rifle forum, they all claim to be rifle experts.

My long distance rifle coach suggests that a .223 must be 1/7 twist, and a 308 must be 1/12 for maximum accuracy as a target rifle.

• The Vietnam AR-15’s had a twist rate of 1/14

• The M-16 has a 1/12 twist for a 55 grain bullet

 

Understanding Barrel twist and rifle bullet accuracy

Ever since the first cannoneer grooved a twist into their cannon barrel, the physics of a spinning bullet has been a topic of heated discussion.  This process of “rifling” and the rate of spin of the bullet obviously influences accuracy.
 
Some say that the twist rates are not all that important:

"The U.S. Army Wound Ballistic Research Laboratory conducted terminal performance testing using 5.56 mm 55 gr M193 FMJ ammunition fired in 20” barrels of 1/14, 1/12, 1/9, and 1/7 twist rates. No difference in terminal performance was noted between shots made with the different twists."

There is a gyroscopic stability factor (Sg) that also influences bullet accuracy.  If a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight.

Fast and slow twists

In general, the lower the twist, the less spin and the “faster” the bullet velocity.  A 1-in-7 twist rate is called “fast” while a 1-in-14 twist rate is called “slow”.

• Fast twist:  A “fast” twist rate can cause bullets to “over-spin” and fall apart from the centrifugal force.  For example, a 1-in-7 twist with a 3,200 ft/sec muzzle velocity will result in a bullet that spins at over 300,000 rotations per second, creating huge centrifugal force.
  
  
• Slow Twist:  A 1-in-12 twist is “slow” and will not stabilize a 62 grain bullet

 

There is also a relationship between bullet weight and twist rate in the .223:

•  Superfast twist for super heavy bullets:  1-in-7 can handle 80 grain bullets.
  
  
•  Fast twist for medium weight bullets:  1-in-9 can handle 60-70 grain bullets.
  
  
•  Slow twist for light bullets:  1-in-12 is better for light bullets (55 grains or less)

The CZ-USA web site states this relationship:

“The 527 Varmint models in .223 Rem feature a 1:9" twist barrel which is able to stabilize bullets up to 70 grains, while the 527 American models in  .223 feature a 1:12" twist more suitable to bullet weights 55 grains and less.” . . “1:7.75 is required to stabilize the 80 grain bullets”

At long distances, a bullet is lobbed like a cannonball.  Over long distances a bullet flies aerodynamically, and the spin becomes an airfoil, a source of “lift”.
 
Too much spin and you get “over-predictability”.
 
The optimal twist rate for the barrel is related primarily to the bullet weight (in grains).

Twist rate calculator

Note the bullet twist calculator that uses the classic Greenhill equation:

T' = 150 / L'

It accepts these parameters and returns the optimal twist rate:

• Bullet length (inches)
• Bullet diameter (inches)
• Muzzle Velocity (feet/second)
• Bullet SG values: (11.3 for lead, 8.9 for copper, 8.5 for brass, 7.8 for steel )

Other factors in twist rate

However, there are other factors that influence twist rates.  Here are the factors, listed in order of importance:

• Bullet grain.
• The length of the bullet.
•  Caliber of bullet
•  Length of barrel
•  Amount of powder in the brass (muzzle velocity) (e.g. 300 magnum vs. 308)
• Air Temperature:  The original M-15 (1/14 twist rate) was changed to (1/12) for arctic conditions.

Obviously, it follows that the formula to determine the gyroscopic stability of a bullet: