Crossing the Sound Barrier
As a projectile slows from supersonic to subsonic speed, it passes through the transonic region near the speed of sound, roughly the band from about Mach 1.2 down to Mach 0.8. In this zone the airflow around the projectile changes character rapidly, and stability that was solid at higher speed can be disturbed.
Why Airflow Becomes Unstable
Above the speed of sound a projectile pushes a well-defined shock wave ahead of it. As it decelerates toward Mach 1, that shock structure weakens and shifts position along the projectile's body. The point where aerodynamic pressure acts can move as the flow reorganizes, and this shifting pressure can nudge the projectile off its stable orientation.
Drag Behavior Near Mach 1
The drag coefficient peaks sharply right around the speed of sound, so a projectile experiences its highest aerodynamic resistance per unit of speed while transitioning. This is part of why velocity and trajectory predictions become less certain in the transonic band, and why drag models that fit well at supersonic speed can mismatch here.
Loss of Gyroscopic Margin
A projectile that was comfortably stable at the muzzle has a stability factor that generally improves as it slows in the supersonic regime, but the transonic flow disturbances impose new overturning influences. If the projectile's gyroscopic stability margin is thin, these disturbances can induce increased yaw, wobble, and a loss of precision as it crosses into subsonic flight.
Design Response
Projectiles engineered for very long range are shaped with long ogives, boat-tails, and carefully controlled profiles to pass through the transonic region with minimal upset. A high ballistic coefficient also pushes the distance at which the projectile slows to transonic speed farther out, so many shooters simply try to keep their effective range inside the supersonic zone.
Worked Example
Imagine a match projectile launched at 2,950 feet per second, where the local speed of sound is about 1,120 feet per second. It stays comfortably supersonic through the early range, but somewhere around 1,200 to 1,300 yards its velocity falls to roughly Mach 1.2 and it enters the transonic band. A well-designed low-drag projectile may still track predictably through this zone, while a less refined shape can begin to show enlarged, erratic groups at that same distance.
A Common Misconception
It is often assumed that any projectile automatically destabilizes and tumbles the instant it goes subsonic. Many well-designed projectiles transition smoothly and remain stable well into subsonic flight. Whether a given projectile is upset depends on its shape and its remaining gyroscopic stability margin, not simply on crossing the speed of sound.