What happens when you shoot an AK-47 under water? This!
This video shows that while it is possible to shoot a gun under water, but we would advise against it.
The slow motion shows a great deal of hot gases pluming into the water. You can learn a lot from this one.
The oxygen needed to launch the bullet is contained within the bullet, so when firing under water, this sealed cartridge with primer, propellant and bullet work as they are made to work on dry land. The difference is water has more density, so the power of the shot bullet is greatly reduced but still could be lethal at close range.
Again, don’t try this at home, because now you know it’s possible and don’t need to prove it to yourself.