I dont have a physics degree, so I cant speak with any authority on this topic, but while the bernoulli fx may be part of the process here Im pretty sure that its not the dominant effect that causes this space to appear. When a bullet shoots out it creates a pressure wave that pushes on the medium it travels through. Im like 75% certain that this is whats creating the space trailing behind it in water the same way that it does in air. EDIT Heres more good pics of the process. http://laststandonzombieisland.files.wordpress.com/2013/07/uss-iowa-firing-broadside-of-16-inchers-note-the-pressure-wave-from-the-muzzle-blast.jpg EDIT 2 Now I think you may be right. http://en.wikipedia.org/wiki/Supercavitation But I kind of feel like theyre just describing what Im describing, but using different language. Wiki says that the object moves the water fast which lowers the pressure, while Im saying it moves the water fast out of the way which creates an area of low pressure. I dunno what I think about this now.