Re: Twist Rate Affects Ballistic Coefficient
Posted: Thu May 07, 2015 7:01 pm
The relationship between twist and BC is being overly simplified here. They are looking at a three dimensional issue in two dimensions. All bullets wobble (technical term is yaw) to a certain degree when they leave the muzzle. This is caused by a number of factors and depend on the bullet aerodynamics (pitch stability, damping, magnus moment, lift, etc.) The wobble is largely a result of gyroscopic precession of the spinning projectile. If the projectile is spun faster, it will have a higher level of gyroscopic stability and generally less yaw (wobble). Increased yaw results in increased drag. Ballistic coefficient is related to drag. So increased gyroscopic stability (i.e., directly affected by increased twist) should reduce wobble and therefore drag.... sometimes.
One thing that many shooters don't realize is that there is another component to bullet stability - dynamic stability. Having gyroscopic stability is a necessary condition for bullet stability, but not completely sufficient. The bullet also has to be dynamically stable.
Gyroscopic stability is principally affected by the bullet's mass properties and static pitch stability characteristics (which are affected by shape, length and flight Mach number). Dynamic stability is affected by all those other parameters I mentioned above.
The bottom line is, increased twist will increase gyroscopic stability, but will not guarantee dynamic stability. Increased twist will reduce yaw oscillations in some cases, but not always. Both stability factors need to be considered.
One thing that many shooters don't realize is that there is another component to bullet stability - dynamic stability. Having gyroscopic stability is a necessary condition for bullet stability, but not completely sufficient. The bullet also has to be dynamically stable.
Gyroscopic stability is principally affected by the bullet's mass properties and static pitch stability characteristics (which are affected by shape, length and flight Mach number). Dynamic stability is affected by all those other parameters I mentioned above.
The bottom line is, increased twist will increase gyroscopic stability, but will not guarantee dynamic stability. Increased twist will reduce yaw oscillations in some cases, but not always. Both stability factors need to be considered.