Remember the opening scene of Saving Private Ryan, when the Allied Forces were pinned down by heavy machine-gun fire from the bunkers on Omaha Beach, so they jumped over the sides of the boats into the water to avoid being hit by stray bullets? Or perhaps that scene from Mission Impossible: Ghost Protocol, when Ethan Hunt (Tom Cruise) and William Brandt (Jeremy Renner) crash into the water when their car is shot?
There are countless such scenes in movies that show people diving into a body of water, like a pool, canal or even an ocean, to avoid being hit by a bullet. Still, there’s a good deal of mystery about whether being underwater can save one from bullets, due to the varied approach (sometimes being underwater saves you and sometimes it doesn’t) that movies take when dealing with this ‘underwater myth’.
Given that, let’s clear up the uncertainty around this frequent and much-debated cinematic trope once and for all!.
Speed vs. Drag
You likely know that every medium offers some resistance to the movement of an object as it moves through it. For instance, the air that surrounds us offers resistance to our common movements, such as walking or running, as we move through it. It’s just that we have become so used to it that we no longer feel this resistance as strongly as we feel the resistance of water while swimming. This opposing force, or the resistance that a fluid offers against the movement of an object moving through it, is called drag.
It’s the drag that wears you down (and slows you down) when you swim through water; it’s the same force that opposes the forward motion of a bullet. A bullet sails easily enough through the air to hit a target, but as soon as it breaches the surface of water, things change drastically!
Once it enters the water, the bullet has to work extra hard against the liquid’s natural drag to continue moving forward. However, in the case of most bullets, they do not go beyond a few feet. Take a look at the image below (taken from a Discovery Channel documentary of D-Day at Omaha Beach).
The bullet is fired from an MG-42 machine gun (a very powerful firing weapon). The bullet leaves the gun at a staggering 3000 feet/second, yet it slows down and comes to a complete halt within less than a yard (2.5 feet at the most)!
Factors affecting a bullet’s speed and range underwater
As you just saw, drag is the biggest cause of concern for a bullet that wants to go fast and deep underwater. However, is the drag offered by water identical for everything that moves through it?
You see, the drag force depends on a number of factors, including the type of bullet, its velocity when fired, the drag coefficient of the bullet and the time it has spent in water. It also depends on the density of the fluid in question (the fluid is water in this case).
For people who have a thing for formulas, here is the drag equation:
where FD is the drag force, ‘p’ (rho) is the mass density of the fluid, v is the flow velocity relative to the object, A is the reference area, and CD is the drag coefficient.
As you can clearly see from the formula, the drag force (FD) of water is directly proportional to the speed of the bullet (v). This means that the higher the speed of the bullet, the more opposition will be offered by the water against the movement.
Quite frankly, underwater battles are pretty counter-intuitive for movie villains who usually engage in fire-fights in open spaces, don’t you think?
Also, the denser the fluid is, the slower it will get as it moves forward. For instance, the density of water is higher than air, so a bullet will slow down much more quickly in water.
Mythbusters, a popular Discovery Channel show, tested the ‘bulletproof’ ability of water in one of their episodes. After firing into a body of water using different guns (including a 9mm pistol, a shotgun, a supersonic M1 Garand semi-automatic rifle, and a .50 cal rifle), they concluded that being 8 feet underwater could protect you against an onslaught of bullets from most guns. However, in real-life cases, a bullet is rarely shot from a position that’s directly above the water. Hence, if the bullet is shot from an angle of 30 Degrees, then being underwater in the range of 3-5 feet (0.9-1.5 meters) can ensure your safety from most guns.
As it turns out, most movies are scientifically correct when they show the protagonist jumping into a pool of water to escape the barrage of bullets being fired at them by the bad guy. I guess it’s not always a bad idea to believe in Hollywood’s movie magic!