[Paal Gulli]

This jives with data published by Alan C Paulson about 30 years ago based on tests begun in the early 90s.

From the 1992 joint suppressor project conducted in Finland:

"...The development of whiplash noise does not begin abruptly at velocity Mach 1.0, but gradually, when the velocity of a rifle caliber projectile is more than Mach 0.90....

The Finns are big on suppressors because their snipers were so effective with them in the "Winter War" against the USSR in 1939-40. They have a saying, "The silencer doesn't make the sniper silent, but it does make him invisible."

In a nutshell, it isn't about how fast the bullet is going, it's about how fast the air has to move to get out of the bullet's path. As the speed of sound approaches, "compressibility" comes into play and air behaves stiffer, less springy, and moves more abruptly when getting out of the projectile's path.

This also goes to the point that the bullet has absolutely no response to crossing the speed of sound. It responds progressively as it enters and then leaves the transonic region, but absolutely nothing of note happens at the instant the bullet slows to exactly the speed of sound.

[Paal Gulli]

The definition of "transonic" is that the air passing over the object is a mixture of subsonic and supersonic, neither entirely supersonic nor entirely subsonic. Going from supersonic to subsonic (or vice-vera) never happens instantaneously. In the case of any object moving through air that experiences aerodynamic drag (which only leaves out radiation, like radio and light waves, but includes bullets), the only way to get from super-or-sub-sonic to the other is through the transonic region.