Miller twist rule is a mathematical formula derived by Don Miller to determine the rate of twist to apply to a given bullet to provide optimum stability using a
rifled
In firearms, rifling is machining helical grooves into the internal (bore) surface of a gun's barrel for the purpose of exerting torque and thus imparting a spin to a projectile around its longitudinal axis during shooting to stabilize the proj ...
barrel.
[Miller, Don. ]
How Good Are Simple Rules For Estimating Rifling Twist
', Precision Shooting - June 2009 Miller suggests that, while
Greenhill's formula works well, there are better and more precise methods for determining the proper twist rate that are no more difficult to compute.
Formula
The following formula is one recommended by Miller:
where
* m = bullet mass in grains
* s = gyroscopic stability factor (dimensionless)
* d = bullet diameter in inches
* l = bullet length in calibers
* t = twist rate in calibers per turn
Also, since one "caliber" in this context is one bullet diameter, we have:
where
= twist rate in inches per turn, and
where
= bullet length in inches.
Stability factor
Solving Miller's formula for
gives the stability factor for a known bullet and twist rate:
Twist in inches per turn
Solving the formula for
gives the twist rate in inches per turn:
Notes
Note that the constant 30 in the formula is Miller's rough approximation of velocity (2800 ft/sec), standard temperature (59 degrees Fahrenheit) and pressure (750 mm Hg and 78% humidity). Miller states that these values are taken from the
Army Standard Metro but does note that his values are slightly off. He goes on to point out that the difference should be small enough that it can be ignored.
It should also be noted that the bullet density is missing from Miller's formula despite the fact that Miller himself states his formula expands upon Greenhill's. The bullet density in the equation above is implicit in
through the
moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular acceler ...
approximation.
Finally, note that the denominator of Miller's formula is based upon the relative shape of a modern bullet. The term
roughly indicates a shape similar to that of an American football.
Safe values
When computing using this formula, Miller suggests several ''safe'' values that can be used for some of the more difficult to determine variables. For example, he states that a
mach
Mach may refer to Mach number, the speed of sound in local conditions. It may also refer to:
Computing
* Mach (kernel), an operating systems kernel technology
* ATI Mach, a 2D GPU chip by ATI
* GNU Mach, the microkernel upon which GNU Hurd is bas ...
number of
= 2.5 (roughly 2800 ft/sec, assuming standard conditions at sea level where 1 Mach is roughly 1116 ft/sec) is a safe value to use for velocity. He also states that rough estimates involving temperature should use
= 2.0.
Example
Using a
Nosler Spitzer
Spitzer is a surname. Notable people with the surname include:
* Andre Spitzer (1945–1972), Israeli fencing coach and victim of the Munich massacre
* Bernard Spitzer (1924–2014), American real estate developer and philanthropist, father of Eli ...
bullet in a
.30-06 Springfield
The .30-06 Springfield cartridge (pronounced "thirty- aught-six" ), 7.62×63mm in metric notation, and called the .30 Gov't '06 by Winchester, was introduced to the United States Army in 1906 and later standardized; it remained in military use ...
, which is similar to the one pictured above, and substituting values for the variables, we determine the estimated optimum twist rate.
where
* m = 180 grains
* s = 2.0 (the safe value noted above)
* d = .308 inches
* l = 1.180" /.308" = 3.83 calibers
The result indicates an optimum twist rate of 39.2511937 calibers per turn. Determining
from
we have
Thus the optimum rate of twist for this bullet should be approximately 12 inches per turn. The typical twist of
.30-06 caliber rifle barrels is 10 inches per turn, accommodating heavier bullets than in this example. A different twist rate often helps explain why some bullets work better in certain rifles when fired under similar conditions.
Comparison to Greenhill's formula
Greenhill's formula is much more complicated in full form. The
rule of thumb
In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
that Greenhill devised based upon his formula is actually what is seen in most writing, including
Wikipedia
Wikipedia is a multilingual free online encyclopedia written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and using a wiki-based editing system. Wikipedia is the largest and most-read refer ...
. The rule of thumb is:
The actual formula is:
[Mosdell, Matthew. ''The Greenhill Formula''. (Accessed 2009 AUG 19)]
where
* S = gyroscopic stability
* s = twist rate in radians per second
* m = polar moment of inertia
*
=
pitching moment coefficient
In aerodynamics, the pitching moment on an airfoil is the moment (or torque) produced by the aerodynamic force on the airfoil if that aerodynamic force is considered to be applied, not at the center of pressure, but at the aerodynamic center ...
* a = angle of attack
* t = transverse moment of inertia
* d = air density
* v = velocity
Thus, Miller essentially took Greenhill's rule of thumb and expanded it slightly, while keeping the formula simple enough to be used by someone with basic math skills. To improve on Greenhill, Miller used mostly empirical data and basic geometry.
Corrective equations
Miller notes several corrective equations that can be used:
The velocity (
) correction for twist (
):
The velocity (
) correction for stability factor (
):
The altitude (
) correction under standard conditions:
where
is altitude in feet.
See also
*
Rifling
In firearms, rifling is machining helical grooves into the internal (bore) surface of a gun's barrel for the purpose of exerting torque and thus imparting a spin to a projectile around its longitudinal axis during shooting to stabilize the pro ...
References
{{Reflist
External links
Calculators for stability and twist
Bowman-Howell Twist Rate CalculatorMiller Formula CalculatorDrag/Twist Calculator based on Bob McCoy's "McGyro" algorithm
Firearm terminology