Supersonic flight is well behaved. Subsonic flight is well behaved. The narrow band between them, roughly Mach 1.2 down to Mach 0.9, is where the pressure field around the bullet reorganises itself — and where neat trajectory predictions stop matching reality.
What happens in the transition
Above the speed of sound, a bullet pushes a shock wave ahead of it and the pressure distribution along its body is stable. As it slows toward Mach 1, that shock detaches and moves. The centre of pressure shifts, drag rises sharply and then falls, and the whole thing happens over a small change in speed.
Two consequences follow. The drag curve becomes steep and shape-dependent, so a coefficient fitted to a reference projectile describes your bullet less well. And the moving centre of pressure changes the balance between the destabilising aerodynamic moment and the gyroscopic stability the twist provides.
Why groups open up
A bullet in flight is not perfectly aligned with its path; it yaws slightly and that yaw damps out over distance. Gyroscopic stability from the twist is what keeps the nose forward. In the transonic region the aerodynamic forces change while the spin rate has decayed from its muzzle value, so the margin of stability is at its thinnest exactly when the disturbance is at its largest.
Marginally stable bullets can pick up yaw they cannot damp, which increases drag unpredictably and disperses impacts. This is the mechanism behind the classic complaint that a rifle 'shoots great to 800 and falls apart at 1,000'.
A faster twist helps, because it leaves more gyroscopic margin at the moment of transition. This is one of the practical arguments for over-stabilising slightly rather than choosing the minimum twist that works at the muzzle — see spin drift for the other side of that trade.
Why your solver drifts off
A solver applies your ballistic coefficient as a scale factor on a reference drag curve. That works well where your bullet and the reference behave alike. Through the transonic region they diverge most, because the shape of the transition depends on details of the nose and boat tail that a single coefficient cannot encode.
In practice, predictions stay tight while the bullet is comfortably supersonic, begin to loosen around Mach 1.2, and should be treated as indicative rather than exact below it. The G7 model holds together better than G1 here for boat-tail bullets, but neither is a substitute for measured data.
Finding your own transonic distance
- Solve a long trajectory in stepsSet the table out past where you expect to lose supersonic flight, at a fine step — 25 or 50 m.
- Read the retained velocity columnFind where it crosses roughly 1.2 times the local speed of sound. In standard conditions that is around 410 m/s, and lower on a cold day.
- Note that distance as your confidence limitInside it, trust the numbers. Beyond it, expect the solution to need confirming with real impacts.
- Recheck it when conditions changeCold air lowers the speed of sound and thin air reduces drag; both move the boundary. It is not a fixed property of the load.
The app reports retained velocity alongside drop for every row of the table and flags whether each row is still supersonic, so this check takes a few seconds rather than arithmetic.
Planning around it
- Choose the load for the distance. A higher-BC bullet stays supersonic further, which pushes the problem past where you intend to shoot.
- Do not extrapolate a DOPE card through the transition. Data measured at 700 and 800 m does not tell you what happens at 1,100 m; go and shoot it.
- Give the bullet twist margin. Over-stabilising slightly costs a little drift and buys stability at the moment it is most needed.
- True inside the supersonic band. Adjusting your coefficient against impacts taken in the transonic region bakes transonic weirdness into your whole trajectory.
That last point matters for anyone who has watched their solver disagree with the target at extreme range and reached for the BC slider. Truing is a valid technique with a valid domain — see truing — and the transonic region is outside it.
Frequently asked
At what distance does a .308 go transonic? +
For typical 168–175 gr match loads, somewhere around 800–900 m in standard conditions — but it depends on muzzle velocity, ballistic coefficient, altitude and air temperature. Read the retained velocity column for your own load rather than trusting a general figure.
Can a bullet be accurate below the speed of sound? +
Yes. Fully subsonic flight is stable and predictable; it is the transition that causes trouble. Subsonic loads are accurate precisely because they never make the crossing.
Does a faster twist fix transonic instability? +
It helps by leaving more gyroscopic margin at the transition, at the cost of slightly more spin drift and higher stress on the bullet jacket. It does not make the aerodynamics predictable.