From Spud Wiki
Shock heating is a proposed explanation for supersonic velocities in certain high-performance precharged air rifles, because it is impossible for ambient temperature air to accelerate a projectile to faster than the speed of sound.
In shock heating, which only occurs at or slightly above the speed of sound in the gas, the flow of gas in the barrel essentially trades pressure for heat. The now heated gas has a higher speed of sound, allowing it to reach a higher velocity.
Although it is suspected that this shock heating occurs in a large number of weak shocks, or even a continuous 'shock volume', in the current versions of the GGDT physics model it is modeled as a single strong shock immediately behind the projectile. This is done to keep computation time reasonable, and produces fairly accurate predictions.
Although successful, it's disputed if it's the correct interpretation of the phenomenon.