Bullet Stability and Barrel Twist Rates — What You Need to Know

Are you curious about bullet stabilization? Do you understand why bullets can tumble or become unstable if the velocity gets too low or if the twist rate is inadequate. Here is an excellent video from Gunwerks that explains Bullet Stabilization.

This animated video starts by showing the design/shape differences between an older-style rifle bullet and a newer VLD-style bullet with higher BCs (Ballistic Coefficients). Generally speaking, the longer a bullet gets relative to diameter, the more RPM is required for stability. And to achieve that higher RPM you need more barrel twist and/or more RPM. The video illustrates where the Center of Gravity and the Center of Pressure are located. These are farther apart (in relative terms) for a VLD or Hybrid-style, long-nose bullet.

When the bullet is in flight there is an angle of attack. This is exaggerated in the animation for illustration purposes, but it is important to understand the the attack angle affect stability. The rotation rate (Revolutions Per Minute) is a function of bullet velocity as it leaves the muzzle and the twist rate of the barrel. Since long VLD-style bullets need more stability, the barrel twist rate needs to be higher than with shorter, fatter bullets. This is pretty much try for all calibers.

The importance of adequate barrel twist rates for bullet stabilization is further discussed in this next video featuring Bryan Litz of Applied Ballistics:

Bullet Stability and Twist Rates

In this video, Bryan Litz talks about bullet in-flight stability and how to calculate barrel twist-rate requirements for long-range bullets. Bryan explains that bullet stability (for conventional projectiles) is basically provided by the spinning of the bullet. But this spin rate is a function of BOTH the nominal twist rate of the barrel AND the velocity of the projectile. Thus, when shooting the same bullet, a very high-speed cartridge may work with a slower barrel twist rate than is required for a lower-speed (less powerful) cartridge. For match bullets, shot at ranges to 1000 yards and beyond, Bryan recommends a twist rate that offers good stability.