In Saving Private Ryan there are scenes where the soldiers are underwater and bullets are shown going into the water with bubbles trailing behind them.
In films with submarines, the propellers often have bubbles coming out.
Where are these bubbles, or the air inside them, coming from?
In: 5
Ryan wasn’t all that accurate there, at least not for the soldiers deeper underwater. Fast-moving rifle bullets, especially at that very shallow angle, don’t have much penetration in water. Here’s Mythbusters doing it with a [.50 BMG](https://youtu.be/yvSTuLIjRm8), didn’t go far.
Otherwise, fast-moving things in water (or water moving fast through things) can cause cavitation. The bullet leaves a low pressure area around it, and that pressure drops below the vapor pressure of the water, which means the water basically boils and you’re looking at steam, not air.
This has been used on purpose to make torpedoes faster. They’re not travelling through high-friction water, they’re travelling through a low-friction tunnel of steam generated by cavitation. This has also ruined a dam, as they did an emergency flood of the tunnels, water started cavitating at imperfections in the tunnels, and eventually started carving out livingroom-sized rocks.
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