Firing a submerged gun is a completely different thing from firing a gun out of the water with a significant amount of water in the bore. A submerged gun has water pressure all over it as well as inside it. A gun in the open air with water in the bore has a big 14 pounds or so of air pressure on the outside and an obstructed bore on the inside. It is pressure differential that causes the problems with water in the bore. The amount of water in the bore and the pressure the cartridge is working with will determine whether the gun fires safely, ends up with a bulge in the barrel, or destroys the barrel, the action, and/or the shooter. The Army issued plastic caps that covered the muzzle and flash hider on M16s when I was on active duty to prevent damage from bore obstructions like water, mud, ice, and snow. BTW, the smaller the bore for a given pressure, the more likely it is for water to cause problems, in my experience.
ECS
Umm...no, I'm afraid not. Your supposition here is all wrong. And it's an error of magnitudes, as well.
That 14.7 psia difference means absolutely...nothing. Outside of a submerged environment, the pressure inside the barrel is still exactly the same as the pressure outside the barrel...namely atmospheric pressure. If you submerged the gun in 33 feet of water, the pressure inside the barrel would be two atmospheres...and so would the pressure outside the barrel.
The overpressure differential when fired submerged is exactly the same as the overpressure differential in the atmosphere for a barrel full of water, because the static pressures are equal inside and outside the barrel. It's kind of like the question I used to ask young submariners in the Navy: if we double the atmospheric pressure in the submarine, does that mean the submarine can dive twice as deep? The answer is "NO"...the submarine can dive 33 feet deeper if you double the atmospheric pressure inside.
Even if you held the barrel of the gun vertical, that pressure of the column of water at the bottom of the would still be negligable. 20-plus inches of water is an insignificant amount of pressure when compared to the 33 feet required for a column of water to reach 14.7 psi at the bottom of the column.
The danger comes solely from the presence of an obstruction, namely that provided by water, in the barrel. Water does have an inertial component to it and, when struck with sufficient speed and force, does exhibit some characteristics of a solid, what with it being an incompressable fluid.
CAN firing a gun with a barrel full of water cause damage to the gun?
The answer is "YES".
WILL firing a gun with a barrel full of water cause damage to the gun?
The answer is "DEPENDS".
That "DEPENDS" encompasses a wide variety of factors, including (but not limited to), barrel thickness, barrel shape, barrel construction (including design and materials), type of ammunition fired, velocity of ammunition fired, breech construction, etc.
Firing a modern small caliber rifle under water? Probably not. Lots of videos on youtube to support this.
Firing a small caliber pistol under water? Again, probably not. Lots of videos on youtube to support this as well.
Firing a large caliber magnum pistol underwater? Maybe. Haven't seen any such videos on youtube...perhaps because people aren't willing to drop the cash in the risk. .44 Magnum, TC in various high powered calibers...that kind.
Firing a high powered rifle under water? Maybe. I suppose this depends on one's definition of "high powered". Personally, I don't consider the .223 to be such a cartridge. 30-06, definately. .50 BMG, definately. But I haven't seen any such videos of these calibers underwater.
Firing a shotgun underwater? Definately. Very easy to see why, when you consider the large internal barrel diameter and thin barrel walls when compared to a rifle barrel.
http://www.youtube.com/watch?v=cxZwV2u2zyU
Bottom line:
Don't do it just because you think it's safe or someone else has told you or shown you that they've done it. Barrel obstructions aren't joking matters, whatever the source.
Drain the barrel and practice good habits to keep obstructions from happening.
