The AK-47 can be considered the Casio watch of the gun world. It takes a licking, and keeps on ticking, except that by ticking we of course mean firing round after round of ammunition. The automatic machine gun is based on a gas recoil system that uses the hot expanding gases of a bullet being fired to automatically rechamber the next round, and surprisingly, it appears to work even better under water.
The folks at Smarter Every Day sunk an AK-47 in an aquarium and, using a high-speed camera, they discovered that the weapon happily still fires while under water. Not only that, it actually reloads itself faster, because H2O can't be compressed. But while the weapon appears to operate more efficiently while submerged, the bullets are also slowed, so unfortunately it's considerably less lethal for hunting mermaids and sharks.