A rifle bullet speeding down range goes through three basic transformations, namely:
Due to it’s speed while supersonic a bullet forces the air out of its path, creating a shock wave that travels ahead of the bullet. Bullets travelling at these speeds do not suffer any major stability problems as long as the inertial stabilizing effects are greater than the aerodynamic de-stabilizing effects. The bullet will fly point first.
When the bullet slows to transonic speed, i.e. when the bullet slows to about 1340 feet per second, it approaches the speed of sound and the sound barrier. That is a bad place for anything to fly.
From the attached diagram it can be seen that the shock wave created during Supersonic flight travels ahead of the bullet and now starts moving closer to the point of the bullet creating instability and increasing the YAW (twisting or oscillation of a moving object about a vertical axis).
SHOCK WAVE STARTING OUT AHEAD OF THE BULLET THEN SLOWING DOWN TO BEHIND
As the bullet slows further the shock wave encapsulates the bullet, creating even further stability problems. For bullets that are spin-stabilized in particular the effect that approaching the sound barrier has on a bullet (as it flies near Mach 1) is that it has a de-stabilizing effect. The centre of pressure moves forward, which causes the over-turning moment on the bullet to increase.
The question then is “Does your bullet have enough gyroscopic stability to overcome the increasing dynamic instability that’s experienced in the transonic speed zone?”
Some bullets do this better than others. Typically, bullets that are shorter and have shallow boat-tail angles will track better through the transonic range. Bullets that are longer can experience a greater range of pitching and yawing in the transonic range that will depress their ballistic coefficients at that speed to greater or lesser extents depending on the exact conditions of the day. These are some of the factors that makes it very difficult to predict your trajectory and path of bullets through the Transonic speed range.
Bullets that make it through the Transonic flight stage will now find it easy going. In subsonic flight the shock wave moves behind the bullet and ceases to have a de-stabilising effect, and therefore it is possible for bullets that are not too unstable to right themselves and continue flying nose first. This is largely due to the ratio of spin to velocity being high allowing this to happen.
Another interesting factor is that the drag factor on the bullet in subsonic flight is about 70% less than during supersonic and transonic flight. All in all this is a good place for bullets to be and where they are at their happiest.
An example 250 grain .338 bullet’s flight path:
Figure 01: Barrel twist required to stabilize the bullet.
Figure 02 Bullet’s Stability factor shown at various speeds.
As shown from around 1500fps destabilizing forces increase, coming to a head at around 1100fps (the sound barrier). Once it goes through the sound barrier and into subsonic flight it is possible for the bullet to rights its self.
There is a very good chance that at around the speed of sound (1100fps) where the SG goes below 1.0 that this particular bullet will become too unstable and tumble, becoming unable to correct itself in the subsonic range.