Calculate bullet stability (Miller SG) from your barrel twist rate, or find the twist a bullet needs, from its weight, diameter, length and velocity.

Miller rule for conventional bullets. 1.5 or more is the usual target for full stability.

For example 0.308 in for .308 Win or 0.224 in for .223 Rem.

Overall bullet length (not the cartridge). Bullet makers list it; measure with calipers if not.

Enter 10 for a 1:10 twist. A smaller number is a faster twist.

Atmospheric conditions
Standard is 59 °F (15 °C). Blank uses standard.

Local station pressure, not sea-level weather pressure. Standard is 29.92 inHg (1013.25 hPa). Blank uses standard.

Twist Rate Stability Formula

This calculator uses the Miller twist rule to estimate a bullet’s gyroscopic stability factor or the twist it needs, and also shows the simpler Greenhill rule below as a rough check of the twist rate needed for stable flight.

T = (C D²) / (L)

Where:

Variable Meaning Practical note
Twist rate Barrel pitch, measured as distance per one full rotation Often written as 1:7, 1:9, 1:10, etc.
Bullet diameter Projectile diameter Use the same length unit as bullet length
Bullet length Projectile overall length Longer bullets generally need faster twist
Greenhill constant Empirical constant used in the rule The calculator’s Greenhill check uses 150 (180 above 2,800 ft/s)

Important interpretation: a smaller twist number means a faster twist. For example, 1:7 spins a bullet faster than 1:10 because the bullet completes one revolution in less barrel travel.

Rearranged Forms

If you already know two values, the equation can be rearranged to solve for the third:

D = √((T L) / (C))
L = (C D²) / (T)

How to Use the Calculator

  1. Choose whether to find the stability factor for your barrel twist or the required twist for a target stability.
  2. Enter the bullet weight, diameter, length and muzzle velocity, picking the unit beside each field.
  3. Enter your barrel twist (10 for 1:10), or keep the target stability of 1.5.
  4. Optionally change the air temperature and station pressure; blank uses standard conditions.
  5. Read the Miller result, plus the Greenhill rule twist shown as a rough check.

How to Interpret the Result

If this changes Required twist tends to Reason
Bullet length increases Get faster Longer bullets are harder to stabilize
Bullet diameter increases Get slower Diameter appears squared in the Greenhill equation
Twist number decreases Spin rate increases The bullet makes a full turn over a shorter distance

In practical terms, this means:

  • Long, slender bullets usually require faster-twist barrels.
  • Shorter bullets can often be stabilized with slower twist rates.
  • Two bullets of the same caliber may need different twists if their lengths differ significantly.

Example

For a bullet with diameter 0.308 in and length 1.5 in, using the calculator’s default Greenhill constant of 150:

T = (150 × 0.308²) / (1.5) ≈ 9.49 in / rev

That result corresponds to an estimated twist of about 1:9.5. In real barrel offerings, that usually means choosing the nearest practical twist specification.

Units and Conversion Notes

  • Diameter and length must match units. If diameter is entered in millimeters, length should also be in millimeters.
  • The output twist rate will be in the same base length unit per revolution.
  • If you switch from inches to millimeters or centimeters, the numeric twist value changes, but the underlying barrel requirement does not.

What the Greenhill Rule Captures Well

  • Fast estimates for typical rifle projectiles
  • Reasonable first-pass barrel twist selection
  • The strong influence of bullet length relative to diameter

Limitations of This Method

  • It is a rule of thumb, not a full aerodynamic model.
  • It does not directly account for exact bullet shape, center of mass, center of pressure, jacket/core construction, or atmospheric conditions.
  • Very high-velocity, low-drag, monolithic, or unusually shaped bullets may need more refined stability analysis.
  • Because the Greenhill check uses C = 150, it should be treated as an estimation tool rather than a final barrel-selection authority.

Common Mistakes

  • Using bullet weight instead of bullet length. Greenhill is driven by diameter and length.
  • Mixing units, such as entering diameter in inches and length in millimeters.
  • Assuming a larger twist number is faster. It is actually slower.
  • Ignoring that similar calibers can require different twists because bullet profiles and lengths vary.

Quick Reference

Result style Meaning
1:7 Fast twist; one full bullet rotation every 7 inches
1:9 Moderately fast twist; common for many rifle applications
1:12 Slower twist; generally better suited to shorter bullets

Frequently Asked Questions

Does a faster twist always mean better accuracy?
Not necessarily. The goal is adequate stability for the bullet being used. Too little twist can cause instability, while excess twist is usually less harmful than insufficient twist but is not automatically optimal for every setup.

Why does bullet length matter more than bullet weight here?
Because stability is tied heavily to the bullet’s geometry. Heavier bullets are often longer, which is why weight is sometimes used as a shortcut, but length is the more direct variable in this formula.

What does inches per revolution mean?
It is the distance the bullet travels down the barrel to complete one full spin. A barrel marked 1:8 means one revolution in 8 inches of travel.