When a bullet leaves a firearm, its trajectory through air is predictable, allowing it to travel considerable distances. The moment it enters water, its behavior changes dramatically. Water’s immediate impact on a bullet’s speed and path drastically limits its travel. This rapid deceleration means a bullet’s effective range underwater is often just a few feet.
Water’s Resistance Explained
Bullets stop quickly in water due to fundamental differences in the properties of water and air. Water is approximately 800 to 1,000 times denser than air, meaning it contains far more molecules packed into the same volume. As a bullet pushes through this denser medium, it encounters more resistance. Water is also significantly more viscous than air, roughly 50 times more so, contributing to its resistance to movement.
This combined resistance is due to hydrodynamic drag, a force opposing an object’s motion through a fluid. Hydrodynamic drag increases with the fluid’s density and the square of the object’s velocity. A fast-moving bullet experiences significant drag upon entering water, rapidly converting its kinetic energy into heat and turbulence. This is similar to running through syrup compared to open air; the thicker medium creates more opposition.
Bullet Behavior Upon Water Entry
Upon impact with water, a bullet changes rapidly. Its high initial velocity meets significant hydrodynamic drag, causing rapid deceleration, often reducing its speed by half within the first five feet. This sudden deceleration can generate cavitation, where a temporary vapor-filled bubble forms behind the bullet due to the low pressure it creates.
The forces exerted by water often cause a bullet to destabilize and tumble end-over-end, or yaw, rather than maintaining a straight path. This tumbling increases the surface area exposed to water, increasing drag and accelerating energy loss. High-velocity bullets often deform or fragment upon water entry due to the pressure and forces acting on their structure. Fragmentation results in multiple smaller pieces, which lose energy quickly.
Key Factors Determining Underwater Distance
Several bullet characteristics and initial conditions influence how far it travels underwater. The bullet’s shape plays a role, as streamlined designs encounter less resistance than blunt-nosed ones. Even an aerodynamically efficient bullet designed for air experiences significant drag in water.
The caliber and mass of the bullet also affect its penetration; heavier, larger-caliber rounds often retain momentum for slightly longer distances, assuming they don’t fragment. High initial velocities, common in rifle rounds, often lead to faster deceleration and fragmentation upon water impact because drag force is proportional to the square of velocity. This means slower-moving pistol rounds sometimes penetrate further than high-velocity rifle rounds before becoming non-lethal, as they are less prone to breaking apart. Environmental factors like water temperature or salinity have minimal impact on bullet travel compared to water’s density and viscosity.
Dispelling Common Myths
A common misconception, often perpetuated in movies, is that submerging oneself in water offers complete protection from gunfire. While water rapidly slows bullets, it does not render them harmless instantly. Bullets remain dangerous and can be lethal even at short distances underwater due to the rapid transfer of their remaining kinetic energy.
Tests have shown that supersonic rifle bullets, including those from powerful .50-caliber firearms, typically disintegrate or become non-lethal within three feet (about 0.9 meters) of water. Slower pistol rounds, such as a 9mm, might retain enough energy to be lethal up to six to eight feet (1.8 to 2.4 meters) underwater, as they often resist fragmentation better. The angle at which a bullet strikes the water’s surface also matters; bullets fired at shallow angles (less than 11 degrees) are more likely to ricochet or break up, while a more direct angle allows for greater initial penetration.