In this video at 7:15 you see a rifle fired underwater: https://www.youtube.com/watch?v=cp5gdUHFGIQ Apparently even before the bullet leaves the barrel, you see a cavitation trace coming out. The author explains it with the water being pushed out of the barrel by the bullet, forming a fast water stream. The fast stream supposedly creates an under-pressure (Bernoulli) which leads to cavitation. How exactly does this work? Is the fast water in the stream evaporating? Already in the barrel or when it gets outside? Or is it the slow surrounding water that gets sucked inwards that evaporates. Are there possible alternative explanations for this first bubble trace (e.g. shock wave)? I don't quite understand the Bernoulli explanation, in a situation with just two fluid parts in relative motion. Since movement is relative, how does one know which fluid part is actually moving, and therefore must have the lower pressure?