Every supersonic bullet eventually slows down to the speed of sound. What happens on the way through is the single most important thing to understand about long-range shooting, and it is the reason this calculator shades a band on every chart.
The speed of sound is not a constant
The local speed of sound depends almost entirely on air temperature:
a ≈ 20.05 × √T metres per second, with T in kelvin.
At the ICAO standard 15 °C that is 340.3 m/s. At −20 °C it is 319 m/s; at +30 °C it is 349 m/s. So the same load crosses Mach 1 at a different distance on a January morning than on a July afternoon — a 30 °C swing moves the crossing point by tens of metres. Pressure and humidity barely matter for the speed of sound (they matter a great deal for air density and therefore drag).
What the transonic band is
By convention this site uses:
| Regime | Mach |
|---|---|
| Supersonic | above 1.2 |
| Transonic | 0.8 to 1.2 |
| Subsonic | below 0.8 |
Different sources draw those lines slightly differently — 0.75 or 0.85 at the bottom, 1.2 or 1.3 at the top. The lines are a convention, not a physical constant; what they bracket is real.
Above Mach 1.2 the flow around the bullet is fully supersonic and there is a stable, attached shock system riding on the nose. Below Mach 0.8 the flow is entirely subsonic and well behaved. Between the two, the flow is mixed: pockets of supersonic flow appear and disappear on the bullet’s shoulders, and the bow shock detaches and moves.
Why accuracy degrades there
Three things happen at once as the bullet decelerates through the band:
- Drag rises very steeply. Look at the standard drag tables: G7’s drag coefficient more than triples between Mach 0.8 and Mach 1.05. That rise is why the velocity curve on the chart bends sharply at that range.
- The centre of pressure moves. As the shock detaches and travels rearwards, the point where aerodynamic force effectively acts shifts along the bullet. The bullet’s static margin — the distance between centre of pressure and centre of gravity — changes, and with it the pitching moment the spin has to fight.
- Yaw damping weakens. A spin-stabilised bullet responds to a disturbance with a coning motion that normally decays. In the transonic band that damping can go to zero or negative, so a small yaw induced at the boundary is no longer suppressed — it persists, or grows.
The result on paper is vertical stringing and a group that opens out of proportion to the distance. This is not a defect in the calculation. The drop numbers past the transonic entry are still the best estimate, but the bullet’s own dispersion around them grows.
The classic illustration is the .308 Winchester at 1000 yards: a 175 gr match load fired at 790 m/s is transonic somewhere around 800–900 m depending on air temperature, which is precisely why 1000-yard .308 shooting is considered hard and why 6.5 mm cartridges took over that discipline. Their bullets stay supersonic past the distance.
Two practical rules
- Know your transonic distance and treat it as a soft ceiling. If a match goes to 900 m and your load goes transonic at 800 m, the answer is a different load, not a better wind call.
- Bullets that never go transonic don’t have the problem. That covers two very different cases: a bullet that stays above Mach 1.2 all the way to the target, and a bullet that starts below Mach 1 and never crosses.
Subsonic loads are a special case
A .308 subsonic load leaves the muzzle at roughly 310–320 m/s. At the standard speed of sound that is about Mach 0.90–0.94 — under the sound barrier, but technically still inside the 0.8–1.2 aerodynamic band. So why are subsonic loads well behaved?
Because the problem is not sitting near Mach 1, it is crossing it. A subsonic load never has an attached bow shock to lose, so nothing detaches and nothing shifts. It decelerates gently into fully subsonic flow. What it gives up instead is enormous: energy scales with velocity squared, so a 320 m/s bullet carries roughly one sixteenth the kinetic energy of a 1280 m/s one of the same weight, and drop past 150 m becomes severe — metres, not centimetres.
There is also a margin question. Because the speed of sound rises with temperature, a load that is comfortably subsonic at 25 °C can be uncomfortably close to Mach 1 at −25 °C, where the sound speed drops to about 315 m/s. Subsonic loads are deliberately loaded well under the barrier to keep that margin in cold weather.
And no, nothing here is hypersonic
Hypersonic conventionally starts at Mach 5, about 1700 m/s at sea level. The fastest commercial rifle cartridges — .220 Swift, .22-250 — leave the muzzle around 1200 m/s, roughly Mach 3.5. Tank APFSDS penetrators reach about Mach 5. No small arm is hypersonic, and any product description claiming otherwise is using the word decoratively.