In the context of bullets and firearms, "transonic" refers to the phase of flight when a bullet is traveling at a velocity that is close to the speed of sound, or it is transitioning from supersonic (faster than the speed of sound) to subsonic (slower than the speed of sound). The speed of sound varies depending on environmental conditions but is roughly around 1,125 feet per second (343 meters per second) at sea level, in dry air, at room temperature.

Here's a breakdown of the different phases of bullet flight:

1. Supersonic: When a bullet is traveling at a velocity significantly higher than the speed of sound, it is in the supersonic phase. During this phase, a shockwave forms around the bullet, creating a distinct "crack" or "sonic boom" as it passes. Supersonic bullets typically have good stability and accuracy.

2. Transonic: As the bullet's velocity decreases due to air resistance and drag, it eventually reaches the transonic phase, where it is approaching the speed of sound. In this phase, the bullet may experience turbulence and instability as it transitions from supersonic to subsonic flight. This can affect accuracy and consistency.

3. Subsonic: Once the bullet's velocity drops below the speed of sound, it enters the subsonic phase. In this phase, the bullet no longer generates a shockwave, and the flight is generally more stable. Subsonic bullets are often used for suppressed firearms or for reduced noise in specialized applications.

The transonic phase can be challenging for bullet stability and accuracy because the aerodynamic forces acting on the bullet change as it approaches the speed of sound. Manufacturers and ammunition designers take these factors into account when developing bullets and loads to maintain good accuracy through the transonic region.

Shooters who are concerned about the transonic region may choose bullets and loads carefully, use ballistic calculators to estimate the range at which bullets will go transonic, and consider these factors in their shooting and hunting strategies. Understanding bullet behavior in the transonic phase is particularly relevant for long-range shooting where bullet stability and precision are critical.