The air infront of the projectile. When you compress a gas it creates heat. The projectile is moving so fast it super heats the air infront of it as it compres

The air infront of the projectile. When you compress a gas it creates heat. The projectile is moving so fast it super heats the air infront of it as it compresses it. Also normally things that go the speed of this projectile (mach 6 I believe) are way up in the sky where the air is less dense. So theres a whole lot of air down here for it to shove out of its way. The explosion is from the rapid expansion of the superheated gas after leaving the barrel.