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When the Mythbusters tested it out, a pool of water stopped a 50. cal sniper rifle under 3 feet. Other weapons tend to go through water more easily, but high-velocity bullets just explode.

Why is that? Why can water stop most bullets, and why it can't stop certain types that effectively?

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    $\begingroup$ The faster-moving bullet experienced a greater stress when it hit the water, so it disintegrated early. Once that happened it was no longer a single bullet but a bunch of lighter pieces. $\endgroup$ – Mike Dunlavey Aug 17 '17 at 19:10
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There are a couple ways to consider this situation.

A somewhat simpler explanation that doesn't account for everything would be the drag equation: $$ F_{D}\,=\,{\tfrac 12}\,\rho \,u^{2}\,C_{D}\,A$$

where $F_D$ is drag force, $\rho$ is fluid density, $u$ is relative velocity, $C_D$ is the drag coefficient for the specific shape and speed, and $A$ is projected area in the direction of travel.

Air is almost 800 times less dense than water. This means that going the same speed through air and water, a bullet will experience approximately 800 times more force in the water.

Because of the $u^2$ term, increasing velocity increases the force dramatically, which is why faster bullets are more likely to break.

Another factor is the shape, which will change the value of $C_D$. A more aerodynamic bullet would be less likely to explode on impact (though speed is likely to be a bigger factor).

A more complicated explanation (and probably just as important or more important) is the compressibility of water. Water is not very compressible, and when you strike it at very high speeds, it may not initially behave as fluidly as you may like/expect. It could act more like a solid surface as the bullet impacts. To analyze this would be somewhat complex and would involve analyzing transient effects and compressibility.

This is a bit more out of my wheelhouse, but something like the water hammer equation may be relevant (especially if you just flip the way you consider it, with the object approaching the water instead). This equation accounts for the equivalent bulk modulus, which describes the compressibility of the fluid. The sudden impact of fluids can be quite forceful.

See also Why is jumping into water from high altitude fatal? and related links.

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