Firing a gun under water

[gmw]

Okay, I've Googled and seen slow motion video etc. of people firing guns under water*, so I do know it's possible, but I still have a question (and given how much interest the last similar topic inspired this seemed like a good place to ask it).

The set-up: A military automatic rifle such as might be found on a U.S. naval vessel - in this case a destroyer. (I assume that they would have some such weapons on board wouldn't they? Anyone know specifically what we might find?) In a scene I'm editing I was thinking of having one character to show another how the gun is used while they are under water (I'm not interested in hitting anything with the bullets, I'm just interested in having the gun fire).

The thing is, we're not talking shooting a few feet down in a swimming pool in this scene, the characters are some hundreds of feet beneath the surface (you'll have to read it to find out how this can be ). Will it fire or might it explode in their hands?

* Amusing side note: search in Duck Duck Go for "firing guns deep under water" gives links talking about "Smoke on the Water" and "Under the Gun" by Deep Purple not very far down the list. Ah, that takes me back.

[avantman42]

Originally Posted by gmw

The set-up: A military automatic rifle such as might be found on a U.S. naval vessel - in this case a destroyer. (I assume that they would have some such weapons on board wouldn't they? Anyone know specifically what we might find?)

This blog post might help with that part, but exactly which small arms are in use will depend on when your story is set. (When is it set?)

I have no idea about firing guns underwater.

[crich70]

Since the gunpowder is between the bullet and the casing when the firing pin is struck by the hammer I'd guess it would fire. Of course since water is thicker than air it would probably have limited range. And the deeper you would go the less light you would have so you might have trouble seeing your target. Your shooter could become part of the menu if he wasn't careful as well since sharks can smell even a little bit of blood at a good distance. And anything in the area would be fair game for them to nibble on (including the shooter). I'd think it would have to be a personal firearm as well rather than a rifle or something like that. Most guns on a navel vessel would probably be under lock and key unless needed. Have you thought of using a spear gun instead? Much easier to get hold of I'd think and the shooter might even be able to get away with shooting someone with it and claiming it was an accident.

My knowledge of firearms is next to non-existent if you're asking "underwater" questions, but Mythbusters might have your answer - they don't go deep-sea levels, but it should be doable to work out the math on your own. Mythbusters provides a lot of details.

[VydorScope]

It will fire how ever your range would be EXTREMELY short and it will not continue to fire, the water will create to much resistance and the automatic functions will often fail. There are lots of videos on youtube of people trying this kind of thing for "torture" testing guns. Look them up and get a feel for what happens.

[Graham]

Originally Posted by EatPrayRead

My knowledge of firearms is next to non-existent if you're asking "underwater" questions, but Mythbusters might have your answer.

Interesting detail about the handgun not ejecting the round due to the pressure, preventing you from firing the next shot. Story potential there.

Graham

[ApK]

Originally Posted by Graham

Interesting detail about the handgun not ejecting the round due to the pressure, preventing you from firing the next shot. Story potential there.

Graham

Fascinating. Glock apparently makes a handgun specifically designed to operate fully submerged:
http://www.youtube.com/watch?v=COea9JqJ7SM

I never served aboard ship, so I can't speak with any authority on what kind of thing you'd actually find on a destroyer, but certainly we have some tin can sailors here?

[JSWolf]

If you can find it, Mythbusters did an episode where they were testing firing a gun under water.

[teh603]

Originally Posted by JSWolf

If you can find it, Mythbusters did an episode where they were testing firing a gun under water.

Was it as un-scientific as the one where they tried to say it was safe to shoot a firearm with a plugged barrel?

[ApK]

Originally Posted by teh603

Was it as un-scientific as the one where they tried to say it was safe to shoot a firearm with a plugged barrel?

The premise of "Mythbusters" has always made me cringe.

The idea is apparently "if these two mid-budget special effects guys can't do it, then NO ONE CAN!"

I have always though the show would be more accurately called "Myth-Slightly-More-Information-Providers"

It's entertaining, but I would steer far clear of it for anything you want to call 'research.'

[gmw]

avantman42 - it is "contemporary". ie. approximately now.

As I said in the OP, I've seen the videos - they're very pretty, there's even a man that makes "flowers" from shooting hollow points under water. (There's even an article that suggests that parts of an AK-47 work better under water.) But things change when you go deep. The pressures get high very quickly. There must be some point at which the pressure of the water will slow down the bullet so much that pressure from exploding round (if it will fire at this depth) will stress the gun. Or is the pressure so evenly great that gun can still withstand the explosion and perhaps fire the bullet a few inches. My maths isn't bad, nor even my basic physics, but I'm not up to working out how great depths may effect the workings of an automatic rifle.

The idea that the gun may jam when trying to eject the empty is interesting. That on it's own may be enough for me to change what I had in mind - as it does seem very likely. It was only an idea I had to explain something that happens later, I may have to find another way.

[ApK]

Originally Posted by gmw

But things change when you go deep. The pressures get high very quickly. There must be some point at which the pressure of the water will slow down the bullet so much that pressure from exploding round (if it will fire at this depth) will stress the gun. Or is the pressure so evenly great that gun can still withstand the explosion and perhaps fire the bullet a few inches. My maths isn't bad, nor even my basic physics, but I'm not up to working out how great depths may effect the workings of an automatic rifle.

From one of the video comments:
"Hydrostatic pressure doesn't quite work that way because it is pushing equally on all sides of the firing pin. A parallel example is that it does not take 4x the force to swim at 100ft as it does right under the surface even though pressure is 4x as great. What matters with drag is the density of the fluid, which fortunately for water does not significantly change with depth."

I'm confident he is correct.
When I dive, my neoprene wet suit compress and gets a bit stiffer, and the air cavities in my body get squeezed, but my muscles and joints continue to work just fine. The valves and switches on my equipment are no harder to operate at depth than at the surface.

ApK

[avantman42]

Originally Posted by gmw

avantman42 - it is "contemporary". ie. approximately now.

In that case, judging by the blog post I linked to earlier, I'd say the small arms likely to be found on a destroyer are M9 pistols, M4 carbines, M16 rifles, M240 machine guns and M249 machine guns. That's assuming the blog post is accurate about everything from pistols to machine guns being carried.

[gmw]

Originally Posted by ApK

From one of the video comments:
"Hydrostatic pressure doesn't quite work that way because it is pushing equally on all sides of the firing pin. A parallel example is that it does not take 4x the force to swim at 100ft as it does right under the surface even though pressure is 4x as great. What matters with drag is the density of the fluid, which fortunately for water does not significantly change with depth."

I'm confident he is correct.
When I dive, my neoprene wet suit compress and gets a bit stiffer, and the air cavities in my body get squeezed, but my muscles and joints continue to work just fine. The valves and switches on my equipment are no harder to operate at depth than at the surface.

ApK

Thanks. That all makes sense. If we assume that there are no air pockets inside the mechanism of the gun that might collapse at depth (and so possibly distort some element before firing), then I can accept that the firing mechanics of the gun may still work.

One of the things mentioned in some of the articles about firing under water is that most bullets are sufficiently water proof that water doesn't enter the case. However I doubt if the case is water proof to hundreds of feet, I wonder if it will just let water in and so even out the pressure, or whether it might collapse at some point with possibly unpredictable results.

[gmw]

Originally Posted by avantman42

In that case, judging by the blog post I linked to earlier, I'd say the small arms likely to be found on a destroyer are M9 pistols, M4 carbines, M16 rifles, M240 machine guns and M249 machine guns. That's assuming the blog post is accurate about everything from pistols to machine guns being carried.

Ah, yes, the M16 fits what I had in mind. Thanks for the link.