Here is a quote by Ernst Mach (from a public lecture):
"If the projectile moves faster than sound, the air ahead of it cannot recede from it quickly enough. The air is condensed and warmed, and thereupon, as all know, the velocity of sound is augmented until the head-wave travels forward as rapidly as the projectile itself, so that there is no need whatever of any additional augmentation of the velocity of propagation. If such a wave were left entirely to itself, it would increase in length and soon pass into an ordinary sound-wave, travelling with less velocity. "
What he's saying there is that the speed of sound ahead of a ballistic projectile actually increases due to the effect of compression -- which raises the temperature of the air in front of the projectile. The formula is of course:
$$c_{ideal}=(\gamma R T)^{0.5}$$
The rest is a continuation of the quote for the sake of context (the public lecture was about the double report of ballistic projectiles from artillery):
"But the projectile is always behind it and so maintains it at its proper density and velocity. Even if the projectile penetrates a piece of cardboard or a board of wood, which catches and obstructs the head-wave, there will, immediately appear at the emerging apex a newly formed, not to say newly born, head-wave. We may observe on the cardboard the reflexion and diffraction of the head-wave, and by means of a flame its refraction, so that no doubt as to its nature can remain."