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A bout one month ago, YouTube series Smarter Every Day not only showed us what it was like to shoot an AK-47 underwater, but also showed us that the gun fires faster while underwater. Though one might assume the water would either slow the firing down or prevent it completely, it turned out that the water creates a large amount of pressure in the chamber, which in turn put more force on the gun’s piston. Now, Smarter Every Day brings us part two of the series, wherein series host Destin films the shot at 27,450 frames per second, as well as teaches us all about cavitation.
After the gun fires, the bullet tears through the water, leaving a pocket of air in its wake. As seen in the super slow motion, the water quickly fill back into the pocket, and kind of bounces when the sides hit each other, creating oscillation.
Thanks to the two different levels of pressure — the bubble’s expansion and the water it’s expanding against — the bubble expands and contracts until there’s no more kinetic energy left from the pressure.
The pockets of air that come out of the gun upon firing are actually three different pockets. The fuzzy one is caused by the gunpowder shooting out of the gun, one is a water jet, and the smooth one is caused by the bullet’s vapor trail (all of which can be seen at around the 8:50 mark in the above video).