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Soviet Underwater Firearms

warisboring.com

79 pointsvinnyglennon30 comments

Re: the last gun in the article:

> The rifle’s effective firing range underwater is about 25 meters at a depth of 30 meters and 18 meters at a depth of 20 meters, according to Russia Today.

Earlier in the article they associate an increase in depth with a lessened range, but with this one it's the reverse? Is that a typo or a nuance of the technology?

I'm only guessing, but it's probably a nuance of the technology. The earlier weapons used cavitation created by the dart's blunt tip to stabilize it and give it range below the surface. The last gun uses a gas bubble to create a supercavitation [1] effect, which based on my quick skimming of the linked Wiki article, seems to be more effective at greater depth.

Anyone with a more thorough (read: any) understanding of fluid dynamics is encouraged to correct or expand this.

[1] https://en.wikipedia.org/wiki/Supercavitation

Ok, let's try: boiling and (super)cavitation are two sides of the same thing. A liquid will turn to vapor (boil) when a physical property of the liquid called "vapor pressure" becomes greater than the actual pressure.

When you normally boil water, you increase the temperature of the water, which increases the water property "vapor pressure" until it becomes equal to atmospheric pressure at 100 C. Or if you are at Mt. Everest, where the pressure is lower, water boils at 71 C.

When cavitation occurs, the opposite is done: you lower pressure until you reach the vapor pressure of the liquid at the current temperature.

In supercavitation, you create a shockwave in front of the bullet/torpedo/propeller which raises and then lowers pressure such that the water boils/cavitates. Why is this good?

Because it lets you put most of the velocity difference between the bullet and the water inside a gas layer. This gas layer has much less resistance to velocity differences than water. Running in water is much harder than running in air, right? So far so good.

So what happens when a supercavitating bullet is fired at greater depth? The pressure is higher, so it's harder to create cavitation, because you have to lower the pressure more to get to the vapor pressure.

Why is that good? Doesn't it reduce the nice gas layer? Yes, it does. My guess is that it also reduces the length of the cavitation bubble behind the bullet, and that reduction in drag is bigger than the increase in drag from a thinner gas layer.

In the older guns, the trailing gas bubble is much shorter because of the projectile shape, and the reduced bubble length at greater depth causes part of the projectile to be outside the bubble, increasing resistance.

You could probably put some decent numerical estimates up by spending an afternoon with White's Viscous Fluid Flow and a steam table.

Might have to do with density of the water which is influenced both by temperature and depth.

Something similar happens with sound but the range is way off.

https://en.m.wikipedia.org/wiki/SOFAR_channel

But the density of water does not vary much by depth (like 1% per 2 kilometers depth). Similar deal for temperature, density lowers by 1% per 20 degrees Celsius (and given that the temp of ocean water stays very much between like >0 and <<35, basically no difference in density.

In fact some of these properties of density of water are what make it such a weird substance, and enable the use of submarines, etc.

Underwater guns are still being developed and marketed. Here's a description of a modern supercavitating Norwegian one: http://www.hisutton.com/Special%20Forces%20diver%20technoloy...

(The book that website promotes is a good buy if you are into this kind of thing.)

No, that is a underwater round, not an underwater gun. Most guns can be used underwater, but range is limited. The bullet in that round extends range from normal rifles. It has the added advantage of being able to be used in air as well (for "over the beach" assaults).

But you are right, they are still developing them. Russia unveiled a new one a couple of years ago (I don't have a link on hand)

The article has pictures and details of a Russian weapon that was new in 2013..

Ah, I recognize these from Depth (http://www.depthgame.com/guide/?command=diver). Didn't know they were real-life firearms.

That day when you find XCom: Terror from the Deep items were actually built by the Soviets? Apparently that's a Monday.

http://strategywiki.org/wiki/X-COM:_Terror_from_the_Deep/Equ...

"fall apart and sink" is a pretty mild way of describing what happens when a rifle round hits water....

These are cool but I wonder if they actually see a lot of use in the wild. SCUBA diver fighting shows up in James Bond a lot but I have to imagine in real life it's more efficient to just cut an enemy diver's air hoses or wait for them to surface and shoot them while they're taking their equipment off.

How are you going to cut their airhose when they're shooting you with an underwater assault rifle?

And if you can get close enough to accurately cut the airhose, why not just stab them in the heart?

> And if you can get close enough to accurately cut the airhose, why not just stab them in the heart?

Because the hose is easier, but your first point applies.

It's not going to be easy getting a knife through a rib cage and it must be hard to get leverage underwater.

> These are cool but I wonder if they actually see a lot of use in the wild.

I wonder if a FOIA request to the Department of Defense requesting "information about any underwater combat engagements between frogmen and/or divers" would result in anything useful?

You might want to look into the story of Lionel Crabb, an SAS frogman suspected of being killed while performing espionage against a soviet warship.

https://en.wikipedia.org/wiki/Lionel_Crabb

That's an interesting read.

I wonder if the DNA is in good enough shape to tell us whether or not the body is his.

Probably more relevant to look at the UK SBS rather than the SAS.

Where did you get the SAS part from?

Special Force: The Untold Story of 22nd Special Air Service Regiment (SAS)

https://books.google.co.uk/books?id=CJ73AgAAQBAJ&pg=PA289&lp...

That doesn't say that he was in the SAS. It just has his name and SAS in the same paragraph.

It says that in the past MI6 needed a person with military skills do something, as was the case with Crabb, so they decided to start cross-training SAS people to do that in the future.

Neither the Wikipedia article nor this endnote says that Crabb was SAS.

I was just hazarding a guess at the question: "Where did you get the SAS part from?"

Ever tried cutting a hose? They're tough, have to be.

Yep. There are knives good at this. I have this one: http://www.amazon.com/gp/product/B000FLTP0A

Nice. My titanium Halcyon struggles with it, but I guess that wasn't a use case they had in mind!

That's significantly harder than shooting them, and they aren't going to not use guns just because you aren't.

Defense in depth.

Nato has also had the H&K P11 since the 70's.

https://en.wikipedia.org/wiki/Heckler_%26_Koch_P11

That weapon is mentioned and linked in the article already though, no?

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