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I am creating a population of human fish that has created a city in an oceanic trench.

There are few things to know about this kind of people and the place where they live:

  • they are a bit stronger than human (the average fish-man is as fit as a professional athlete)
  • they can freely breathe underwater and out of the sea as well
  • depth as no effect on them
  • the city is illuminated by a intensive farming of the same bacteria used by lamprey
  • they have access to all technology of today (it's OK if their weapon can only be made on earth, they have good commercial relationships with classical humans)

I have already thought about melee weapons but I want to know if at this depth, with enough budget on research we could make at least a working harpoon thrower. Even if there is no way that they could manually pull the string themselves, they could at least use it one time.

I don't want a weapon of mass destruction, but things that a single person can use.

And if there is a way to get a weapon like that, would it be usable outside water or at a lower pressure too?

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  • $\begingroup$ Spear-guns are traditionally used to hunt fish. More powerful versions can be deadly weapons. Tridents and pikes could be used a support weapons. Sharp pointy things can be used underwater. $\endgroup$ – a4android Jan 9 '18 at 5:00
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    $\begingroup$ Lithium or sodium payloads. $\endgroup$ – JDługosz Jan 9 '18 at 5:03
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    $\begingroup$ Who on earth VTC'd this as "unclear what you're asking?" Really? Even before @a4android's edit it's clear what the OP's looking for. (Can't be a light beam, it would defract to readily. Is there an energy beam that can pierce salt water to a reasonable distance? Maybe a sonic device, if you can keep it tight enough.) $\endgroup$ – JBH Jan 9 '18 at 5:10
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    $\begingroup$ @JBH Agreed. A weapon that works underwater. What's hard about that? Seemed easy enough to see that's what the OP was asking about. $\endgroup$ – a4android Jan 9 '18 at 5:33
  • $\begingroup$ Sonar riot control less-than-lethal weapons. I wonder if you can get constructive interference to focus the effect on a target without blowing your own eardrums out? $\endgroup$ – Muuski Jan 9 '18 at 12:43
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Never bring a knife to a dart fight.

Heckler & Koch P11

I pity the poor unfortunate soul who takes a hit from this

The HK P11 is a Heckler & Koch pistol designed as an underwater firearm that was developed in 1976. It has five barrels and each fires a 7.62 X 36mm dart electrically. Loading is by means of a five-round case. The design resembles that of a pepper-box firearm.

SPP-1 underwater pistol

darts!

The SPP-1 underwater pistol was made in the Soviet Union for use underwater by Soviet frogmen as an underwater firearm. It was developed in the late 1960s and accepted for use in 1971. Underwater, ordinary-shaped bullets are inaccurate and very short-range. As a result, this pistol fires a round-based 4.5 millimeters (0.18 in) caliber steel dart about 115 millimeters (4.5 in) long, weighing 12.8 grams (0.45 oz), which has longer range and more penetrating power than speargun spears. The complete cartridge is 145 millimeters (5.7 in) long and weighs 17.5 grams (0.62 oz).


Follow the links in the guns' names for more info.

These weapons are from the cold war era. They had a rather short range (not effective over 20 meters underwater or 30 on air). With a bit more tech advancement, and the demand caused by your species living underwater, and your world could have versions with longer range.

Also, torpedoes to do the job of our world rocket and grenade launchers.

And trained war sharks. Just because sharks.

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  • $\begingroup$ More modern versions of these weapons might use supercavitating projectiles. They would have odd shapes to "push" the water aside and crate a bubble for the bullet to move in. Without rocket propulsion you might still double the range from 20 to 40m. $\endgroup$ – Thucydides Jan 9 '18 at 15:49
  • $\begingroup$ @Thucydides do you have a source on that? Supercavitating projectiles will have unstable trajectories after their bubbles collapse, which is a reason they were not used in the past. $\endgroup$ – Renan Jan 9 '18 at 16:24
  • $\begingroup$ There have been a number of articles on self propelled supercavitating weapons (i.e rocket propelled torpedoes) and USN experiments with special bullets which create supercavitating bubbles when fired into or in water. One possible use for supercavitating bullets was to shoot at mines from helicopters. When the bubble collapses, then the bullet's usefulness is at an end, much like reaching maximum range when firing a rifle normally. $\endgroup$ – Thucydides Jan 10 '18 at 5:59
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I would think harpoons would work perfectly well. Resistance to pressure does not translate to resistance to puncturing.

Otherwise, sonic weapons (weaponized lithotripters), or tasers (salt water is conducting, but copper wires are way more so, and most of the charge would be delivered on target).

For longer ranges, I think the only reasonable way to go is torpedoes, but given their speed, they wouldn't work very well as antipersonnel weapons. Possibly, explosive-tipped torpedoes could work, if they had some sort of proximity fuse.

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  • $\begingroup$ Electrical weapons in water, seriously? $\endgroup$ – Feyre Jan 9 '18 at 13:18
  • $\begingroup$ Absolutely. Electric eels and electric rays use biologically generated electric currents as a contact weapon. Add a couple of reasonably insulated wires and two small harpoons and you've got an underwater taser. $\endgroup$ – LSerni Jan 9 '18 at 14:01
  • $\begingroup$ @LSerni no, those are not contact weapons. Electric fish don't need to touch you in order to shock you. $\endgroup$ – Renan Jan 9 '18 at 14:06
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    $\begingroup$ @LSerni the thing about eels is just nitpicking. I think we should focus on the underwater taser - I think it's feasible (+1) with insulated copper cables. An alternative to that would be a thermal tazer, which stuns by deploying chemicals that undergo an endothermic reaction in order to freeze the target. I so want to play Terror From The Deep right now. $\endgroup$ – Renan Jan 9 '18 at 14:17
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    $\begingroup$ @Renan, the thermal taser looks promising. Depending on the creatures' thermal sensitivity, it could even work as a limited area denial weapon. And most cold-blooded creatures get dazed by cold. $\endgroup$ – LSerni Jan 9 '18 at 15:41
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I was wondering if standard firearms cartridges generated enough pressure to be practical to use to drive a speargun at that sort of depth. A bit of googling around gives barrel pressures in the 21Kpsi to 60Kpsi range, for different types (http://www.closefocusresearch.com/calculating-barrel-pressure-and-projectile-velocity-gun-systems) and a water pressure increase of a bit under 14.5psi per 10m depth, or a bit under 14.5Kpsi at 10Km. So that looks OK (but suggests that a slower burning gyrojet style projectile might have problems).

On the other hand, if you're using powdered propellant, at 10Km it's likely going to be crushed into a solid block unless you have enormously thick cartridge walls (and an equally thick projectile at the end). And presumably a firing pin isn't going to work unless you can trigger your primer by a shock wave through the cartridge wall (which is likely too thick to deform).

If you could get a solid propellant charge to work, then things get a lot easier; if there's no air space, you probably don't need the thick walls (as long as the pressure wouldn't trigger the primer).

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  • $\begingroup$ Electric primers are your friend. And it may be possible to have some form of chemical propellant which burns underwater, so the cartridge does not have to "isolate" the propellant, leaving the pressure equalized in an unfired cartridge case. In this instance the case serves to carry the proper amount of propellant and allow for ease of storage and use. $\endgroup$ – Thucydides Jan 9 '18 at 15:56
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A gun with supercavitating self-propelling rounds, something like a Gyrojet, the rounds would either be longer/heavier to have comparable penetrating power to a bullet and/or be tipped with explosives/poison/acid/etc.

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  • $\begingroup$ The bullets would be to big for a handheld gun. You would need a mounted weapon to have those bullets. Also notice that unless they are tipped with explosives and do explode at the end of their trajectory, their trajectories will be unstable (and possibly go random) after the projectile can no longer maintain a supercavity bubble. $\endgroup$ – Renan Jan 9 '18 at 14:05
  • $\begingroup$ Gyrojet rounds were 13mm in calibre, but were only somewhat larger than a conventional .45ACP round. It is quite possible to build rocket powered rounds in any size, with the general caveat that larger rounds can hold more propellant and have longer range/greater kinetic energy on impact. And "handheld guns" have been made to fire up to .50 cal conventional rounds. $\endgroup$ – Thucydides Jan 9 '18 at 15:53

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