Transonic problem[edit]
When the velocity of a projectile fired at
supersonic muzzle velocity approaches the speed of sound it enters the
transonic region (about
Mach 1.2–0.8). In the transonic region, the centre of pressure (CP) of most non spherical projectiles shifts forward as the projectile decelerates. That CP shift affects the (dynamic) stability of the projectile. If the projectile is not well stabilized, it can not remain pointing forward through the transonic region (the projectile starts to exhibit an unwanted
precession or coning motion called limit cycle yaw that, if not damped out, can eventually end in uncontrollable tumbling along the length axis). However, even if the projectile has sufficient stability (static and dynamic) to be able to fly through the transonic region and stays pointing forward, it is still affected. The erratic and sudden CP shift and (temporary) decrease of dynamic stability can cause significant dispersion (and hence significant accuracy decay), even if the projectile's flight becomes well behaved again when it enters the
subsonic region. This makes accurately predicting the ballistic behavior of projectiles in the transonic region very difficult. Because of this, marksmen normally restrict themselves to engaging targets within the supersonic range of the projectile used.
[note 1] In 2015 the American ballistician Bryan Litz introduced the "Extended Long Range" concept to define rifle shooting at ranges where supersonic fired (rifle) bullets enter the transonic region. According to Litz, "Extended Long Range starts whenever the bullet slows to its transonic range. As the bullet slows down to approach Mach 1, it starts to encounter transonic effects, which are more complex and difficult to account for, compared to the supersonic range where the bullet is relatively well-behaved."
[40]
The ambient
air density has a significant effect on dynamic stability during transonic transition. Though the ambient air density is a variable environmental factor, adverse transonic transition effects can be negated better by a projectile traveling through less dense air, than when traveling through denser air. Projectile or bullet length also affects limit cycle yaw. Longer projectiles experience more limit cycle yaw than shorter projectiles of the same diameter. Another feature of projectile design that has been identified as having an effect on the unwanted limit cycle yaw motion is the chamfer at the base of the projectile. At the very base, or heel of a projectile or bullet, there is a 0.25 to 0.50 mm (0.01 to 0.02 in) chamfer, or radius. The presence of this radius causes the projectile to fly with greater limit cycle yaw angles.
[41] Rifling can also have a subtle effect on limit cycle yaw.
[42] In general faster spinning projectiles experience less limit cycle yaw.
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