It's pretty incredible what some people can do with a Phantom Miro 320S high-speed camera and a fairly, extensive understanding of guns. This video is a follow-up to another video Smarter Every Day did where he fired a pistol underwater. In this video, he fires a modified AK-47 underwater to study how a blowback system works underwater versus in the air and study how a gas-operated rifle works when it's filled with water.
According to the video, the gun appears to work even better underwater, despite a slower bullet.
I cannot emphasis this enough: DO NOT TRY THIS AT HOME.
This is the original video (shot with a high speed camera made by Vision Research):