Most conventional bullets fired into water lose their dangerous velocity within just a few feet. Water is roughly 800 times denser than air, so the drag forces acting on a bullet increase dramatically the instant it crosses the surface. The exact distance depends on caliber, bullet shape, muzzle velocity, and the angle at which the round enters the water, but the general picture is consistent: water is remarkably effective at stopping projectiles. That said, the physics involved are more interesting than a simple “bullets stop fast” and have driven decades of military research into making them travel farther.
Why Water Slows Bullets So Dramatically
The core reason is fluid density. When a bullet leaves a barrel in open air, it pushes aside relatively sparse gas molecules. Water molecules are packed far more tightly, so the bullet has to shove aside a much heavier mass with every inch of forward travel. The resistance a moving object encounters in a fluid scales directly with that fluid’s density, which means a bullet faces roughly 800 times more drag in water than in air at the same speed.
That drag is not gentle. Research analyzing high-speed video of bullets entering water found that within the first 20 milliseconds, the bullet’s motion is dominated by what physicists call Newtonian drag, the brute-force resistance of pushing heavy fluid out of the way, rather than the thinner viscous drag that matters more at lower speeds in lighter fluids.1Physics Education. A bullet fired in dry water: an investigative activity to learn hydrodynamics concepts In practical terms, the bullet decelerates violently from the moment it hits the surface. A round that enters water at several hundred meters per second can shed almost all of its kinetic energy in under a meter.
For a military-grade 7.62 mm round carrying about 3,000 joules of kinetic energy, experiments have shown that nearly all of that energy transfers to the surrounding water within roughly one millisecond of entering the fluid.2Journal of Fluids and Structures. Experimental analysis of 7.62 mm hydrodynamic ram in containers One millisecond. That energy does not disappear; it blasts outward through the water as a pressure wave, which is why shooting into a sealed container of water can rupture the container violently. But from the bullet’s perspective, its forward momentum is essentially gone almost immediately.
How Caliber, Speed, and Shape Change the Distance
You might assume that a faster, more powerful round would travel farther underwater. Counterintuitively, the opposite is often true for conventional ammunition. Faster bullets generate more drag because fluid resistance increases with the square of velocity. A round entering at twice the speed does not just experience twice the resistance; it experiences roughly four times as much. This means high-velocity rifle rounds can actually stop in a shorter distance than slower pistol rounds, because they dump their energy more aggressively.
Bullet shape plays a major role as well. Research on ballistic projectiles entering water has shown that the geometry of the bullet tip and the ratio of its length to its diameter are critical factors in how it behaves underwater. Bullets with lower length-to-diameter ratios tend to tumble almost immediately after entering water, while longer, narrower projectiles can maintain a more stable path inside the vapor cavity that forms behind them as they push through the fluid.37th International Symposium on Cavitation. Shallow angle water entry of ballistic projectiles Tumbling is the kiss of death for underwater distance because it massively increases the bullet’s cross-sectional area relative to its direction of travel, which multiplies drag.
Standard military and civilian rounds were never designed to stay stable in water. A typical hollow-point or soft-point bullet will deform, fragment, or begin tumbling almost as soon as it enters the water. Round-nose bullets and full metal jacket rounds fare slightly better, but “better” still means they are losing lethal velocity within a few feet. The general rule of thumb, borne out by numerous controlled tests, is that most handgun rounds become non-lethal within about three to six feet of water, while rifle rounds often lose their danger even sooner due to their higher velocity and tendency to fragment.
What Actually Happens to the Bullet
Water does not just slow a bullet down; it frequently destroys it. Experiments with 7.62 mm ammunition fired into water-filled containers found that the projectile was partially destroyed in every test configuration, regardless of the container setup. The interaction with the fluid alone was enough to break the bullet into fragments.2Journal of Fluids and Structures. Experimental analysis of 7.62 mm hydrodynamic ram in containers This fragmentation happens because the forces on the bullet at high speed in dense fluid are not evenly distributed. Asymmetric pressure causes the bullet to yaw, and once it starts turning even slightly off-axis, the stresses on the jacket and core escalate rapidly until the projectile breaks apart.
Expanding ammunition, like hollow-points, is particularly prone to immediate destruction. These bullets are designed to mushroom on impact with soft tissue, and water hits them with even more force per unit area than flesh does. The hollow cavity at the tip acts like a scoop, catching water and forcing the bullet open violently. Fragments of jacket and lead core scatter in a cone, each piece losing velocity even faster than an intact bullet would. This is one reason why underwater bullet travel is so much shorter than people expect from watching action movies: the bullet often does not survive as a single object long enough to travel more than a body length.
The Angle of Entry Matters More Than You Think
Not every bullet that hits water actually enters the water. At shallow angles, bullets ricochet off the surface. The critical angle at which a bullet transitions from skipping off the water to penetrating it depends on multiple factors, including bullet weight, velocity, shape, and surprisingly, the state of the water’s surface. Research simulating bullet impacts on water has shown that whether waves are present, and the direction those waves are traveling relative to the bullet’s path, change both the critical ricochet angle and whether the ricocheting bullet remains stable or tumbles.4The Journal of Defense Modeling and Simulation: Applications, Methodology, Technology. Assessing the Risk of Bullet Ricochet from Waves
This has real safety implications. A bullet that ricochets off water retains much of its velocity and can travel a considerable distance through the air before striking something. Hunters and target shooters near bodies of water face a genuine ricochet hazard, and firing ranges located near lakes or ponds factor this into their safety planning. A bullet that enters the water at too shallow an angle and bounces is far more dangerous to bystanders than one that penetrates the surface and stops within a few feet.
For a bullet that does enter the water, the entry angle also affects how far it travels below the surface. A round entering at a steep, near-vertical angle encounters the maximum cross-section of water resistance immediately and decelerates fastest. A round entering at a shallower angle may travel a slightly longer path because the effective resistance ramps up more gradually, though the horizontal distance covered underwater is still short. Redesigned projectile geometries, such as those with specifically shaped tips, have been shown to exploit this by creating a vapor cavity after impact that dramatically reduces drag, allowing travel over greater distances underwater.37th International Symposium on Cavitation. Shallow angle water entry of ballistic projectiles Those engineered projectiles are a far cry from what comes out of a standard firearm, though.
Supercavitation and Bullets Built for Water
The short travel distance of normal bullets in water is actually a problem that some militaries have tried to solve. The concept is called supercavitation: engineering a projectile so that when it enters water, it creates a large, sustained bubble of vapor around itself. Inside this bubble, the bullet is essentially flying through gas rather than liquid, which slashes drag enormously and lets it travel much farther than a conventional round ever could.
Supercavitating ammunition works by using a specially shaped nose, called a cavitator, that deflects water outward and creates a low-pressure zone behind the tip. If the projectile is moving fast enough, the water in this zone vaporizes, forming a cavity that can envelop the entire bullet. Instead of pushing through dense liquid, the bullet rides inside a pocket of water vapor, experiencing dramatically less resistance. Military researchers have developed supercavitating projectiles and even underwater firearms designed to fire them. One experimentally verified model uses a 5.7 mm diameter supercavitating round fired from a smooth-bore underwater rifle, and the ballistic model developed from those tests has been validated against real firing data.5Advances in Military Technology. Ballistics of Supercavitating Projectiles
The results are striking. While a conventional bullet might be stopped within a meter or two, supercavitating rounds can maintain dangerous velocity over tens of meters underwater, depending on the specific design. Some military programs have reportedly achieved effective ranges of 60 meters or more for special-purpose underwater ammunition, though specific performance figures are often classified or restricted. The technology has been pursued seriously by several countries, and small arms specifically designed for underwater combat exist, including Russian systems that have been in service for decades.
These weapons highlight an important point: the reason ordinary bullets fail in water is not that water is inherently impenetrable. It is that ordinary bullets were designed to fly through air and are catastrophically mismatched with the physics of moving through liquid. With the right shape, the right materials, and the right velocity, projectiles can be made to travel meaningful distances underwater. The physics allows it; the engineering just has to accommodate the fluid dynamics.
Can You Actually Hide Behind Water?
The short answer from a practical standpoint is that a few feet of water provides extraordinary protection against conventional small arms fire, far more than most solid materials of equivalent thickness. A standard interior wall will not stop a rifle round. Three feet of water will. This is not a recommendation to use swimming pools as bomb shelters, but the physics are real and have been tested extensively.
A person submerged a few feet below the surface in a pool, lake, or ocean is genuinely very difficult to injure with gunfire from above the surface. The bullet’s energy dissipates so quickly that even high-powered rifle rounds are unlikely to cause a lethal wound past about three feet of water depth, and most become harmless well before that. The fragmentation that occurs in water also means the bullet is not arriving as a single, concentrated penetrator; it is arriving as a spray of low-energy fragments, if it arrives at all.
That said, some caveats apply. If the shooter is very close to the water’s surface and the target is just below it, the first few inches of water may not provide enough deceleration, particularly for heavy, slow-moving rounds that resist tumbling slightly longer than light, fast ones. Shotgun slugs, which are massive and relatively slow, can penetrate deeper than lighter rifle bullets before stopping. And at the surface itself, there is almost no protection; a bullet hitting the water at a steep angle still has substantial energy for the first fraction of a second.
Hollywood Versus Reality
Action films routinely show bullets zipping through water in long, visible trails, maintaining enough force to hit targets several body lengths away. The reality is far less cinematic. A bullet entering a pool would create a dramatic splash and a brief trail of bubbles, but the projectile itself would be tumbling, deforming, or fragmenting within inches of the entry point. The visible streak in a pool is mostly cavitation vapor and disturbed water, not a bullet still flying straight.
One persistent misconception is that larger-caliber weapons are more effective at shooting into water. While it is true that a heavier bullet carries more momentum, the increased cross-sectional area of a larger round also increases drag proportionally. The net result is that bigger bullets do not necessarily travel meaningfully farther underwater than smaller ones. In fact, as mentioned, the shape and speed of the bullet matter more than its caliber. A small, streamlined round designed for water entry will outperform a large, blunt round every time.
Another misconception is that you can “shoot through” a body of water to hit something on the far side, like firing through a fish tank to hit a person behind it. For very thin layers of water, like a few inches in a small aquarium, a bullet might retain enough energy to be dangerous on the other side, though it will have been deflected off its original trajectory. For anything resembling a meaningful depth of water, the bullet will not exit the far side with enough velocity to matter. The energy transfer to the fluid is simply too rapid and too complete.
Depth, Salinity, and Temperature
Readers sometimes wonder whether saltwater stops bullets differently than freshwater, or whether water temperature makes a difference. Salt water is slightly denser than fresh water, about 2.5 percent more dense in typical ocean conditions, so it does exert marginally more drag on a projectile. In practice, this difference is negligible compared to the enormous drag water already exerts. You would not notice a meaningful difference in how far a bullet travels in a swimming pool versus the ocean.
Temperature has an even smaller effect. Cold water is slightly denser than warm water, but again, the difference is trivial against the backdrop of water being 800 times denser than air. The state of the surface matters more: choppy water with waves changes ricochet behavior, as discussed earlier, and can also disrupt the initial entry dynamics of a bullet, potentially causing earlier tumbling or deflection.
Depth itself does not change the physics much for the distances involved. Water pressure increases with depth, but the pressure difference between the surface and ten feet down is tiny compared to the forces already acting on a high-speed projectile. A bullet fired into calm water at the surface behaves essentially the same as one that somehow enters the water at a depth of thirty feet. The stopping power of the water is a function of density, the bullet’s velocity, and the bullet’s shape, not the ambient pressure of the surrounding fluid at the depths relevant to this question.