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Because standard ammunition does not work well underwater, a common feature of underwater firearms is that they fire steel darts instead of standard bullets.
The barrels of underwater pistols are typically not rifled. Rather, the fired projectile maintains its ballistic trajectory underwater by hydrodynamic effects. The lack of rifling renders these weapons relatively inaccurate when fired out of water. While underwater rifles are more powerful than underwater pistols and more accurate in out-of-water applications, underwater pistols can be manipulated more easily underwater.
Among the many engineering challenges in designing underwater firearms is that of developing a weapon which can be effective in both underwater and out-of-water applications. The ASM-DT amphibious rifle is an early example of such a weapon. First fielded by the Russian special forces in 2000, the folding-stock ASM-DT amphibious rifle is capable of firing two kinds of ammunition, both of 5.45 mm caliber:
Supercavitating ammunition that functions better in the underwater environment has been developed recently by Defence & Security Group (DSG), a Norwegian company. The Multi-Environment Ammunition (MEA) series supercavitating rifle ammunition, developed and marketed by DSG, is anticipated to be useful for certain special operations, including underwater warfare. These include defensive applications such as diver protection, as well as offensive applications such as neutralization of enemy divers, VBSS (Visit, Board, Search, and Seizure Operations), MIO (Maritime Interception Operations), GOPLATS (Gasoline and Oil Platforms) boarding operations, speedboat interdiction and neutralization, anti-submarine warfare, anti-torpedo operations, and certain counter-piracy operations.
The ballistic characteristics of MEA series ammunition allows the user to fire at an underwater target from above the water, an above-the-water target from below the surface, or from an underwater location at a target which is also underwater. This innovative ammunition allows the user to fire from above the surface into the water at a very low angle of incidence — in some cases as low as two degrees — without fear of ricochet. After entering the water, the bullet will continue its original trajectory. The user does however need to compensate for the refractive index of the water (roughly 1.333 for fresh water at 20 °C).
MEA series supercavitating ammunition is currently available in the following calibers:
The unique underwater capabilities of MEA series supercavitating ammunition allow it to be used for Unmanned underwater vehicle (UUV) applications. The armed UUVs can be used in both offensive and defensive underwater warfare operations. Using the .50 BMG supercavitating cartridge, an armed UUV can potentially destroy steel-hulled underwater objects from a distance of 60 meters, or could potentially hit a target 1,000 meters in the air from a location 5 meters below the surface.