The density is water is approximately 800x greater than the density of air. Drag force (air/fluid resistance) scales with the square of the velocity and with the density of the fluid. This means that a bullet fired underwater experiences 800x more drag force than a bullet fired in air and will only travel a very short distance before losing all its momentum. The bullet can be seen tumbling shortly after exiting the gun, which further increases the resistance to its movement. The pressure waves generated by the firing can be seen rapidly expanding outwards before collapsing backwards along the path of the bullet.
A Mythbusters episode found that for most guns, being only 3 feet underwater would protect you from a gun fired at a 30-degree angle (including a .50-caliber gun with armor-piercing rounds). They confirmed the myth that you can dive underwater to protect yourself from gunfire.
Source: https://youtu.be/OubvTOHWTms (Slow Mo Guys)
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