I'm not trying to start (continue) a debate along the lines of AK vs. AR, really!!  I just read in the maintenance forum about procedures for cleaning an AR after submersion in water.  The advice given was good; should clean up an AR pretty well.  It just seems like a lot of work to have to do just to get the weapon back into action, and in the middle of a firefight you just wouldn't have any time at all.  I'v read/heard stories of Vietcong with AK's hiding under the water, and then coming up and opening fire as the water drains from the muzzle.  I've also seen pictures of SEALS, Rangers, etc. doing water ops, where their weapons (M4's, M16's) are also quite "wet", to say the least, and yet they appear to be functioning just fine.  What gives?  I've owned both weapons systems, but I guess I'm just to chicken to try firing one underwater, or imediately after being submerged.  Has anyone here ever done it?  What happened?

Gunpowder is designed to burn so that the pressure in the bore is fairly stable while the bullet is in the bore.  Of course, this means that it is burning in a situation where there is an increasing volume where it can burn.  (As the bullet goes through the bore, it leaves more volume behind it.)  Powder is made to continue burning as the bullet travels down the bore.  It is trying exert a consistant push to the bullet via tens of thousands of psi.

What this means is that if you plug the bore, even with water, you would be inhibiting the ability of the bullet to travel down it.  The powder will not wait for the bullet to move, and in fact will burn even faster.  All that unreleived pressure will burst the gun in some fashion.  The Gunpowder will function more like high explosive in this case.  Proper function depends on bullet movement at the rate the powder designer assumed.

As far as shooting in the atmosphere right after immersion:

The AR's gas system will be much slower to drain the water because it is smaller and tighter, but your focus should be on caliber not type.

A 7.62 bore is far better at geting rid of water than a 5.56.  Water can freely drain from 7.62, whereas surface tension can hold lots in a 5.56.  Enough can be held that it is a restriction on the bullets movement.  While not like firing underwater, it can still be dangerous.

My two cents......