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.
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
- 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)
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
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
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: