My chronograph tells me that high velocity ammunition is right at the supersonic level in a pistol(1044 fps). Due to some deviation in the ammo, some rounds are supersonic, some rounds are subsonic, even from the same box of ammo. Standard velocity is almost always just under supersonic in a pistol length barrel, but well into supersonic in a rifle. Subsonic is very much below the speed of sound even in a rifle. The problem with barely supersonic ammunition is not that the speed of the bullet is too high--the problem is that the bullet goes through a transition of speed on it's way to the target, passing from supersonic to subsonic. There is some (although not much) turbulence as the projectile passes down through the sound barrier, causing some instability in flight.