http://www.dailymail.co.uk/sciencetech/article-2348158/How-firing-AK47-underwater-actually-make-work-BETTER.html

 

Submerging an AK 47 gun and firing it underwater appears to make it work better, according to high speed footage.

 

The water ahead of the bullet in the barrel is being forced into the gas port an driving the piston even before the round gets passed the port. thus causing the chamber to unlock and begin to open much sooner and with high pressures still in the chamber and hence the pushed primers.

 

 

My theory anyway.

The ballistics don't seem improved underwater. 

Only the force that drives the piston back increases as water does not compress like gas.

Correct, water does not compress. So instead of the mass ahead of the projectile (air) being in a gaseous state that WILL compress as it is forced out of the barrel (and also into the gas port) because the bullet is pushing it. The liquid in the barrel that has just been forced into the gas port will impart far more force on the gas piston causing it to begin retract before the projectile has even passed the gas port.

 

once again that's just my theory.

Only if by 'better' you mean the bullet travels a greatly reduced distance at a greatly reduced velocity.

This is part 2, This crazy bastard did this in his pool. However it is pretty cool.