A lot of movies have a scene with Someone being chased by enemies; Someone jumps into a body of water to escape, swims deep, and catches a bunch of bullets fired from above. This is patent nonsense; water will stop most bullets in just a foot or three, if not a matter of inches. But it seems that if you design your bullets properly, you can get meaningful travel through water measured in *dozens* of feet. Supercavitation forms a thin layer of gas around the projectile so that the projectile seems to be traveling through a gas rather than a liquid, massively dropping drag. In some high speed torpedoes this gas sheath is created by ejecting gas from the nose. But bullets do this by carefully shaping the nose; the shaping causes the water to boil into a vapor.
Just the thing for the Navy to use to take out incoming torpedoes, nearby mines and pesky scuba divers. Now you can use your AR-15 for fishing. Neato.