What causes the smoke to appear after firing a rail-gun?

In this video https://youtu.be/QO_zXuOQy6A. They show a rail-gun firing multiple shots and after each there is a large cloud of smoke. What causes this? Is it condensation from water vapor in the air? Is it heat transfer from the round?
White smoke occurs after a rail-gun fires multiple shots because some of the un-burned fuel or coolant escapes from the barrel. [1] This is caused by either too much fuel being injected into the combustion chamber, or not enough heat to burn the fuel. [2]
Railguns are extremely loud because of the high velocity of the projectiles they fire. [1][2] Because railgun projectiles travel so quickly, they generate a tremendous amount of heat upon firing. [1][3] This heat causes significant pressure inside the railgun, which can cause an "explosion". [2] In railguns that are used on ships, this pressure can be isolated to a sealed chamber, sealing the sound. [3] While in railguns that are used on land, the sound is damped by the atmosphere. [4]