Firing a gun under water

[gmw]

Hmm... 1 atmosphere = approx 15psi, water pressure increases approx 1 atmosphere every 10m (say 33ft), so at a depth of 1000 feet we're talking around 450psi. Rifle chamber pressures are somewhere around 30,000 to 60,000 psi (give or take a pretty big margin). So there should be plenty of pressure left to expel the bullet.

I was wondering about the gas operated auto-load systems these rifles use, but there is this gas operated APS rifle designed for under water use. And this article claims the AK-47 gas operation works better under water (but uses a different system to the M16). Given that even at a 1000ft the water is still not getting close to the chamber pressure there seems reason to hope the systems might still work.

But all this still hinges on whether cartridges have survived the journey to this sort of depth.

[ApK]

I must admit I'm curious....why are your people trying to fire guns at 1000 ft?

[Penforhire]

I'm curious about the magnitude of pressure wave built up ahead of the fired bullet. Water is dense enough that I'd be concerned about exceeding the barrel pressure design limit.

Second curiosity is whether the powder would burn as powerfully as in air. I know the cartridge is generally sealed and so it should ignite if the primer is struck. But it is possible that atmosphere (oxygen) just ahead of the cartridge contributes to gunpowder burn, especially in long-burn-duration rifle cartridges (versus pistol).

[WillAdams]

No, there's a gas seal caused by the bullet being driven into the barrel --- oxygen in the barrel doesn't contribute to combustion --- the gunpowder has only the oxygen in w/in the cartridge for burning.

[jgaiser]

Originally Posted by ApK

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?

Spent 10 years in the U.S. Navy on everything from Amphib ships to aircraft carriers. At that time (mid 60's to mid 70's) every ship had both pistols and rifles. Of course they were mostly WWII leftovers. 45Cal pistols and 30Cal M1 in the beginning and eventually newer stuff. So yes I would guess that the current armament would be what ever the current infantry is carrying.

[Penforhire]

Will, I agree with you on the bullet-to-barrel seal but I think most breeches are not atmosphere sealed (vented to at least reduce 'kaboom' damage). Ah, just had a thought though, the breech doesn't matter because the cartridge brass will seal that end of the barrel during the burn. So indeed combustion must be sufficient with just the atmosphere inside a given cartridge.

Which is also interesting because some cartridges are so full of powder versus others, thinking of "compressed" rifle loads versus, say, the small powder load in .357 Magnum. Must not take much oxygen to burn even a bunch of powder.

[ApK]

Originally Posted by Penforhire

. So indeed combustion must be sufficient with just the atmosphere inside a given cartridge.

As I understand it, smokeless powder (the stuff in modern cartridges) is nitrocellulose, which is its own chemical oxidizer. It does not need atmospheric oxygen in or out of the cartridge.

[Greg Anos]

Originally Posted by ApK

As I understand it, smokeless powder (the stuff in modern cartridges) is nitrocellulose, which is its own chemical oxidizer. It does not need atmospheric oxygen in or out of the cartridge.

Correct.

[gmw]

Originally Posted by ApK

I must admit I'm curious....why are your people trying to fire guns at 1000 ft?

Because that's where they are at the time. This is a contemporary fantasy series, the "people" in this case are not exactly human - which is why they need lessons on how to fire a gun.

[ApK]

Originally Posted by gmw

Because that's where they are at the time. This is a contemporary fantasy series, the "people" in this case are not exactly human - which is why they need lessons on how to fire a gun.

Hmm. I'd read it.

[VydorScope]

I still think you would have a problem with wet bullets not going off. I have no expertise to call on other then lots of youtube videos were after holding a gun under water and trying to shoot, they fail before emptying the clip.

[gmw]

In shallow water the casing (from what I've read) is water proof enough that the propellant never gets wet. In deeper water the question arises: will the case remain water tight?

If so (if the water is kept out), is the case strong enough to withstand the even external pressure of 450psi or might it collapse, and if it collapses what might be the result? Another question that occurs is whether the primer is weaker than the casing, and so might collapse first and fire the charge early?

If not (if water gets in), is water inside the case likely to cause problems with ignition? As I understand it modern propellants won't care about the water if you can ignite it, but it seems feasible that ignition from the primer could be an issue - but I don't know enough about it to be sure.

I think (mostly from what I've read) that most of the instances of guns failing under water has more to do with water interfering with the feed mechanisms (ejection and/or loading) than with the bullets themselves. I haven't watched that many of the videos, only have limited download and relatively slow connection here.

[VydorScope]

Well the vids I am thinking of the gun was in INCHES of water and failed after fairly short time and not because of the feed. If I get time and remember later I will find some of the vids and link them...

Why not uses a weapon designed for underwater use in your story instead of a regular gun? Then you won't have these problems.

[gmw]

Because the gun is just a random one grabbed off the people on the destroyer. (This was just an idea I was exploring, an edit to an existing scene, in the hope of explaining another part of the book. It's not that important to the story, just a detail I wanted to cover. It's all looking a bit iffy, so I may decide to scrap the idea and find another way of achieving what I want.)

ETA: Isn't it amazing how much time you can spend on something that would probably only be a few lines in the book. But if you screw it up then you drop the the reader out of the story possibly destroying pages of effort.

[crich70]

Originally Posted by gmw

Because the gun is just a random one grabbed off the people on the destroyer. (This was just an idea I was exploring, an edit to an existing scene, in the hope of explaining another part of the book. It's not that important to the story, just a detail I wanted to cover. It's all looking a bit iffy, so I may decide to scrap the idea and find another way of achieving what I want.)

ETA: Isn't it amazing how much time you can spend on something that would probably only be a few lines in the book. But if you screw it up then you drop the the reader out of the story possibly destroying pages of effort.

I understand Hollywood has similar problems. There are people whose whole job is to make sure things are as consistent as possible. For example if the hero is smoking and he's just lit a cigarette (far shot) it's their job to make sure that the cigarette isn't 3/4ths burned away (in the close up) a few seconds later. Of course errors do still crop up. The old blind man with the golden idol in "The Ten Commandments" is a classic screw up. He's wearing a Timex wristwatch!