The Bullet That Swims

The U.S. Navy and Norwegian defense group Nammo have developed a super-cavitating bullet that can travel through water. While traditional ammunition is either stopped or deflected when it hits water, Nammo’s 30 mm Swimmer (APFSDS-T MK 258 Mod 1) swims straight through it.

Nammo says the U.S. Army and the U.S. Navy are now adopting the Swimmer for use from many platforms, including the US Army’s latest addition, the Stryker variant known as “Dragoon.”

The Swimmer is a sub-caliber kinetic energy penetrator made out of very heavy materials that use the force of the impact rather than explosives to punch through armor. Traveling at speeds of more than one kilometer per second, the energy generated by the impact melts the armor of the vehicle into a fluid and the arrow “swims” through the armored side of the vehicle. In the case of the Swimmer, the force of the arrow is sufficient to defeat anything except main battle tanks. 

Nammo’s penetrators, both for vehicles and other types of armor piercing ammunition, are made out of a super tough tungsten alloy, also known as wolfram. More than two times as heavy as steel, it has the second highest melting point of any element, making it ideal for use in armor piercing ammunition. For some ammunition types, including the 25 mm APEX for the F-35, tungsten is mixed with carbon, creating tungsten carbide. This makes the penetrators harder, but also more brittle, allowing them to fragment once they have penetrated the armor, causing added damage inside the target. 

Design engineer Jan Hasslid says the Swimmer is unique because of the combination of powerful armor penetration and its ability to swim straight through water. “This effect has until now been considered impossible to achieve by ammunition fired from air through water. As demonstrated by a number of popular science TV programs, traditional ammunition is either stopped or deflected when it hits water. In a worst case scenario, a projectile could hit the surface, bounce off and hit something else.”

The Swimmer avoids the ricochet in water problem through the use of a super-cavitation nose design. This means that the projectile creates a bubble of steam around itself big enough to pass through, substantially reducing the friction that stops traditional ammunition. “This enables the Swimmer to be used in defense of either ships or coastal areas against submerged and surface mines, small underwater vehicles, torpedoes and even small fast attack crafts that might be concealed by waves,” says Hasslid. “This is valuable not only for naval vessels, but also for land vehicles defending harbors, bridges or other key locations.”

Nammo is now turning its attention towards developing a new type of programmable 30 mm ammunition that can be used against the kind of targets where the Swimmer is less effective. Based on the same technology as the 40 mm grenades that have been demonstrated against drones, this would allow fighting vehicles to program their ammunition to explode either in the air, or at a specific point inside a target.