The guys on Mythbusters decided to find out whether swimming under water would protect you from gunfire. They tested a 9mm pistol, a 12 gauge shotgun, an AR-15, a .30 cal. M1 Garand and a .50 BMG rifle.
The started out with a 10-foot-high water tank in which they lowered a block of ballistic gelatin. The 9mm penetrated the gel completely down to seven feet but at eight feet it barely penetrated the front of the gel block. Then they tried the shotgun. The pressure from the shotgun blast blew out the sides of the tank and the shockwave alone was enough to blow a cavity out of the back of the gel block, however the pellets did very little damage. Chances are, in open water the shotgun would not be very effective.
They moved to a swimming pool to test the AR, M1 and .50BMG. In all three cases the bullets disintegrated before striking the gel, which was only about 14 inches below the surface, but at a slant line distance of about 10 feet from the shooter. So, when shooting at a subject swimming underwater a 9mm pistol is much more likely to cause a serious or fatal wound than even a .50BMG.
The started out with a 10-foot-high water tank in which they lowered a block of ballistic gelatin. The 9mm penetrated the gel completely down to seven feet but at eight feet it barely penetrated the front of the gel block. Then they tried the shotgun. The pressure from the shotgun blast blew out the sides of the tank and the shockwave alone was enough to blow a cavity out of the back of the gel block, however the pellets did very little damage. Chances are, in open water the shotgun would not be very effective.
They moved to a swimming pool to test the AR, M1 and .50BMG. In all three cases the bullets disintegrated before striking the gel, which was only about 14 inches below the surface, but at a slant line distance of about 10 feet from the shooter. So, when shooting at a subject swimming underwater a 9mm pistol is much more likely to cause a serious or fatal wound than even a .50BMG.