Twist Rate Calculator

What Is a Twist Rate Calculator?

Barrel twist rate describes how far a bullet must travel down the bore to complete one full revolution, written as a ratio such as 1:7 for one turn in seven inches. A faster twist spins the bullet harder, which is what gyroscopically stabilizes long, heavy projectiles. The Twist Rate Calculator converts muzzle velocity and twist rate into bullet spin rate in revolutions per minute.

How the Calculation Works

The bullet spins once per twist length of barrel travel. Muzzle velocity in feet per second is first converted to revolutions per minute by multiplying by 60 seconds per minute and 12 inches per foot:

$$RPM = \frac{v_{fps} \times 60 \times 12}{twist_{in}} = \frac{v_{fps} \times 720}{twist_{in}}$$

Where:

• v_fps is muzzle velocity in feet per second
• twist_in is the twist length in inches per revolution, such as 7 for a 1:7 barrel
• RPM is the resulting spin rate in revolutions per minute

Velocity may also be entered in meters per second, in which case it is converted at 3.28084 ft/s per m/s.

Worked Example

A 5.56 mm round leaves a 1:7 barrel at 2,600 fps. The spin rate is 2,600 × 720 / 7 = 267,429 RPM, roughly 4,457 revolutions per second. A slower 1:12 barrel firing the same round would spin it at 156,000 RPM, which is often not enough to stabilize a long bullet.

For related flight and ballistics tools, see the True Airspeed Calculator and Wind Correction Angle Calculator.