
Soviet Underwater Firearms - vinnyglennon
http://warisboring.com/articles/underwater-firearms-are-a-thing-and-russia-is-really-into-them/
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FLTexcalibur
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?

~~~
rosser
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
_super_ cavitation [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](https://en.wikipedia.org/wiki/Supercavitation)

~~~
semi-extrinsic
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.

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willvarfar
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...](http://www.hisutton.com/Special%20Forces%20diver%20technoloy%20-%20DSG%20Supercavitating%20Ammunition.html)

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

~~~
steve19
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)

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

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

~~~
ethbro
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...](http://strategywiki.org/wiki/X-COM:_Terror_from_the_Deep/Equipment)

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dsfyu404ed
"fall apart and sink" is a pretty mild way of describing what happens when a
rifle round hits water....

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evan_
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.

~~~
mattmanser
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?

~~~
maaku
> 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.

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

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jemfinch
Defense in depth.

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dh997
Nato has also had the H&K P11 since the 70's.

[https://en.wikipedia.org/wiki/Heckler_%26_Koch_P11](https://en.wikipedia.org/wiki/Heckler_%26_Koch_P11)

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

