Ordinary guns don't fire underwater worth a darn. Bullets quickly slow to a stop in water, a medium 8 times more dense than air.
The tungsten-tipped CAV-X bullet from Virginia’s DSG Technologies has a nose that creates a small air bubble as it moves through the water. The bubble greatly reduces drag on the bullet, a supercavitation effect that has been part of torpedo design since World War II and is now a feature on some types of boats.
DSG is also selling to other governments, Leonhardsen said. He declined to identify them. He did say those governments were testing how well the bullets performed when fired from helicopters into the water. One potential target: submarines. In tests performed by DSG, the .50-cal. bullets can travel 60 meters through water or penetrate 2 cm of steel through 17 meters of water.
Fans of 19th century science fiction French phenom Jules Verne also came up with a way of usefully firing a gun under water in his 1875 classic 20,000 Under The Sea:
"Besides M. Aronnax, you must see yourself that, during our submarine hunt, we can spend but little air and but few balls."
"But it seems to me that in this twilight, and in the midst of this fluid, which is very dense compared with the atmosphere, shots could not go far, nor easily prove mortal."
"Sir, on the contrary, with this gun every blow is mortal; and, however lightly the animal is touched, it falls as if struck by a thunderbolt."
"Because the balls sent by this gun are not ordinary balls, but little cases of glass. These glass cases are covered with a case of steel, and weighted with a pellet of lead; they are real Leyden bottles, into which the electricity is forced to a very high tension. With the slightest shock they are discharged, and the animal, however strong it may be, falls dead.