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I looked at this question, and a very similar but not duplicate question popped into my head. Although most of the answers mention melting or force, they all use a rifle as the preferred weapon. Not knowing enough to answer, I held my tongue. I think a shotgun would still be feasible given the correct wadding, but I'm not sure.

So to help my curiosity, the OP of that question, and any future reader looking to incorporate this into their stories, here goes.

How would an ice gun work? What other materials could be used as a dissolvable or untraceable bullet?

Note: I'm not looking for bullets that would be hard to find, such as those that shatter on impact but ones that essentially vanish. Please assume a modern day setting.

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  • $\begingroup$ "At what range would an ice block used in a shotgun slug... be?". I'm not sure what you're asking here. Are you asking the ranges at which such a projectile might be effective? Or at what range would such a projectile stop existing (ie, melt)? $\endgroup$ – Lord Dust Sep 9 '16 at 15:23
  • $\begingroup$ Also, do you specify the shotgun shell in an effort to keep the slug from shattering? $\endgroup$ – Lord Dust Sep 9 '16 at 15:24
  • $\begingroup$ Shattering, no. Provided a better wad, it could easily ramp the speed correctly to prevent a total failure, and as for the effectiveness, I'd like both if possible. $\endgroup$ – Anoplexian Sep 9 '16 at 15:28
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    $\begingroup$ This was tried on Mythbusters. It doesn’t work because ice, even using modifiers, jackets, etc., is just too light weight. $\endgroup$ – JDługosz Sep 9 '16 at 16:21
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    $\begingroup$ Anyone who wants the ATF knocking on their door, can take this and run with it : salt. $\endgroup$ – Mazura Sep 11 '16 at 1:36

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A salt gun would also work, provided that some models are around. However, if you do want to kill someone with it, you might want a saboted salt round. As an added bonus, if you hit the guy, you will cause a lot of pain. And if the guy still has parts of his wits around him, he would probably try to get most of the salt out to reduce the pain. If he does not, the salt would most likely dissolve into the body after some time (dependent on where you hit).

Some extra credit to @Mazura who posted this in a comment before I put this answer up, but which I saw after I posted this.

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    $\begingroup$ That is just evil. $\endgroup$ – NuWin Sep 15 '16 at 8:03
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    $\begingroup$ rock salt is already used in shotguns and it does not dissolve entirely, the salt actually creates a clot around itself preserving pieces of it unless the person survives. $\endgroup$ – John Jul 18 '17 at 4:50
  • $\begingroup$ I always thought salt is specifically used as a nonlethal ammunition $\endgroup$ – Andrey Jul 19 '17 at 20:43
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You asked about other substances. If it looks like shattered shards of a substance the coroner already expects to find then it is effectively disappeared.

Would a bullet of bone be viable to make it untraceable? The bullet is pretty lightweight but maybe workable. Bonus points if it is the victim's own bone to survive DNA analysis. comments suggest this is not viable: too easily detected

Instead of just ice, blood plasma can be frozen solid. Again, DNA bonus points.

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    $\begingroup$ Oooo I REALLY like this idea. It could also be used to frame someone else for the crime. "Whose DNA did you find? Well it was odd, there are multiple DNA traces in the body that we aren't sure about. Well, run the tests again." That would definitely serve that purpose! $\endgroup$ – Anoplexian Sep 9 '16 at 16:36
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    $\begingroup$ @Anoplexian Just as a note, when doing genetic assays they usually know what medium they are pulling DNA from. It's unlikely bone DNA will accidentally contaminate, say, blood DNA. $\endgroup$ – Nathaniel Ford Sep 9 '16 at 17:00
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    $\begingroup$ @NathanielFord Right, but blood plasma in blood? $\endgroup$ – Anoplexian Sep 9 '16 at 17:17
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    $\begingroup$ Shards of bone would be suspicious if the victim wasn't shot in a bone. $\endgroup$ – rek Sep 9 '16 at 18:04
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    $\begingroup$ @Anoplexian and SRM, Bone would still be traceable, as Mythbusters determined when they brought a pathologist on the show. $\endgroup$ – Frostfyre Sep 9 '16 at 18:05
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Answering the question in a different way, you could construct a gun that fires a dry ice pellet.

Dry ice sublimates from solid to gas above -78 so it would evaporate rather than disolve. You could also use it as the propellant, and it is slightly denser than water ice so would be a marginally better projectile.

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  • $\begingroup$ Nice, but would it make it out the end of the gun before sublimation occurs due to friction with air? And in what sort of way would this weapon be constructed? $\endgroup$ – Anoplexian Sep 9 '16 at 20:57
  • $\begingroup$ I don't think it would expire that quickly, especially if the whole barrel was cooled. imagine a set up similar to a musket with the dry ice projectile wedged in until a certain pressure built, add water behind to encourage the sublimation, rather than gunpowder. maybe some kind of iris that can be opened to release the projectile. Or just use an air gun. $\endgroup$ – Whinja Sep 9 '16 at 21:18
  • $\begingroup$ youtube.com/watch?v=sVaMxSBCKxA first demonstration would probably be similar to the size of the pellet $\endgroup$ – Whinja Sep 9 '16 at 21:25
  • $\begingroup$ Wouldn't dry-ice leave a mark on the body as it evaporates since it is really cold? $\endgroup$ – SMS von der Tann Sep 11 '16 at 12:42
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    $\begingroup$ @SMSvonderTann the question just asked about disappearing, not leaving any traces (since a dead body is itself a pretty big trace). $\endgroup$ – PyRulez Jul 18 '17 at 11:46
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Why not use a dart gun?

You could fire the dart which carries a very powerful toxin. Once it has impacted with the target, the dart could withdraw the needle and drop to the ground. With today's tech using a little robotics and AI, it could possibly crawl a little way away and burn itself to a crisp, or perhaps even crawl away under a door or even further.

On the body, you would only have a small pin prick wound, and if the AI is smart enough, the "dart-bot" could take itself well away from the crime scene.

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"Regular" water ice itself is unlikely to be of value. The disappearance of the projectile is unlikely to be an advantage, since even though the projectile cannot be found, it will be obvious that someone was attacked with a projectile weapon. There might be some narrow corner cases where this technology might carry some kind of legal advantage, but overcoming the enormous issues of projectile storage, projectile coherence on launch, penetrative and stopping power, range, and accuracy just to name a few would be prohibitive.

On the other hand, other materials might be of use. Consider Molly's fletcher pistol from the William Gibson novel Neuromancer. It's a compressed air-powered pistol that fires flechettes made of a frozen neurotoxin. Range, accuracy, and stopping power are not good, but the deadliness of the round makes up for that. Most importantly for your requirements, the projectile is very small, and dissolves very quickly in the body. It won't immediately be obvious that either a projectile was used, or the nature of the toxin that actually caused the target's death. For the purposes of fiction, you could posit all kinds of fantastic chemical substances to use as projectiles.

If stricter science is required, you might try arsenic, some allotropes of which sublimate fairly readily at room temperature and pressure. It seems readily amenable to being made into a compound that could be made to disappear, while imparting a deadly payload to a target.

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  • $\begingroup$ Interesting idea! I would like to however note that the point of a disappearing bullet is to remove forensic evidence as to the bore of the barrel, especially in rifle and pistol ammunition. This does however remind me of a dart gun, which would be an interesting way kill someone. $\endgroup$ – Anoplexian Sep 9 '16 at 20:56
  • $\begingroup$ Indeed. And the Fletcher also provides a lack of permanent projectile and more. In particular, the poor penetration of an ice bullet means that your biggest upgrade from that concept is something more deadly. $\endgroup$ – Lord Dust Sep 9 '16 at 21:50
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You're going to need a large gun to do this effectively:

Make your bullet out of sodium. It's actually a sabot round but the sabot material is highly flammable so it burns up before falling to the ground (shoot from a high position to give it time to be destroyed--think of magician's flash paper for the sort of thing I'm picturing.) (The sabot is to shield it from the worst of the effects of firing. If you can build a big enough air rifle you can dispense with the sabot.)

The melting point is a problem, you're going to lose some of your bullet to melting when you fire it and you'll lose some in flight. It's also lightweight, it's not going to penetrate it's target much more than it's own size so you'll need a decent chunk to pull this off. The range won't be long, both because of melting and because you can't rifle the barrel.

The spent bullet will burn up and it will react with the body--but note that sodium is normally in the body, this is going to be much harder to detect than if you used something exotic. If they look hard enough they'll figure it out, though.

There's really no need to get this fancy, though. Take an ordinary rifle, fit a bag over the mechanism to catch the spent cases. Shoot your target then replace your rifle barrel.

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  • $\begingroup$ I would expect an explosion of some order when the sodium projectile enters the body as sodium and water don't go together well. $\endgroup$ – SMS von der Tann Sep 11 '16 at 12:45
  • $\begingroup$ @SMS You'll get a reaction but I don't think you'll get an explosion. The water inside the human body isn't loose, the reaction will be slowed down considerably. $\endgroup$ – Loren Pechtel Sep 11 '16 at 20:24
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My answer didn't use gunpowder, posited air or rail guns, and it was posted 33 minutes before your question. Oh well.

My alternatives here answer the question of impact that disappears, but it probably fails your modern day setting requirement. Plus I'm thinking totally out-of-the-box and ignore dissolvable or untraceable bullet.

Sound Waves

All these suggestions probably require longer exposure on the target that a mere gunshot would at current technology levels.

Long Range Acoustic Device (LRAD)

While termed non-lethal, LRAD makes people run into tear gas to avoid the pain. So the target may run off a high place and fall to their death?

If you managed to trap the target and then bombard them with LRAD for 5 minutes there would be hearing damage and possible heart, lung and internal injuries.

High Intensity Ultrasound

https://en.wikipedia.org/wiki/Sonic_weapon#Research lists lung and intestinal tissue damage (in mice), heart rate atrial flutter and bradycardia.

Just design an emitter without the safety protocols: Low Frequency Sonar at 184+ dB or a focused ultrasound beam of 1 mW/cm² SPTA.

If you can get at the target's bedroom while they're sleeping you can probably kill them with sound.

Misused Directed Energy Weapon

https://en.wikipedia.org/wiki/Directed-energy_weapon

These would leave evidence but given nobody dies of this stuff at the moment, a coroner would be puzzled by the delivery if not the cause of death.

Fry your target with a real microwave or an ADS.

Frozen Blood Weapon

I love @SRM's answer, especially if you harvested the target's own blood first.

Liquid Nitrogen's boiling point is (−195.79 °C (77 K; −320 °F)) so store the weapon in a portable flask.

Easiest method would be for a street vendor to stab the target repeatedly with an icicle made of his own blood. The local Nitrogen Ice Cream cart takes a darker turn.

Alternatively we're back to custom air rifles (more likely air cannon) or rail/coil guns with mesh/foil sabot.

Surveillance and Counter Surveillance

All of these methods require a good understanding of the target's movements and habits. But if you're trying to kill someone in an unusual way, I suspect that's part of the process.

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Frozen mercury would do nicely. Mercury is dense enough that it will maintain sufficient energy when striking the target to do suitable damage, yet melt into an unmarked blob. And if the impact doesn't kill them, they'll die from massive mercury poisoning.

The trouble with ice bullets is - no density, so they don't maintain much energy when they're moving. The primary reason bullets are made with a lead core is weight - so that they impart a lot of energy to whatever they strike.

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How about slower moving projectiles? Wouldn't an ice arrow be deadly enough? Or how about a "paintball round" that's filled with something extremely reactive, like hydrofluoric acid?

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  • $\begingroup$ Part of this question, an important part, is the concept of a disappearing bullet. A HF round will leave distinct chemical traces as well as obvious signs of how it attacked the flesh of the victim. However, this would be a particularly nasty way of harming/killing someone. $\endgroup$ – a4android Jul 20 '17 at 2:09
  • $\begingroup$ @a4android That's true. Maybe polonium instead. $\endgroup$ – Salmoncrusher Jul 20 '17 at 19:24
  • $\begingroup$ Polonium is radioactive. It will be something like a normal bullet. Normal bullets could be used instead. The radioactivity might be a dead giveaway. Admittedly someone would need to run a radiation detector over the wound and bullet fragments. A chemical assay would show it was polonium. Polonium works better as a poison. $\endgroup$ – a4android Jul 21 '17 at 7:29
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The effectiveness of the bullet is determined by its kinetic energy on impact, which is measured by half the product of the mass times velocity squared:

$$E_k = \frac{mv^2}{2}$$

To impart velocity in a traditional gun, a controlled explosion is used. The sudden increase in pressure would cause the water ice to shatter. Therefore, you need to increase the speed without subjecting the ice bullet to excess pressure.

You could do this with a rail gun and a metallic sabot.

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    $\begingroup$ I think mythbusters tried a metal jacket. Still no good. Meanwhile a pingpong ball at supersonic speed could barely break skin. So more speed doesn’t help much when mass is too low. $\endgroup$ – JDługosz Sep 9 '16 at 16:23
  • $\begingroup$ Or density, more to the point. A 5.56 round isn't exactly what I'd call massive. $\endgroup$ – Lord Dust Sep 9 '16 at 16:49
  • $\begingroup$ When I said increase speed I was meaning in the initial acceleration of the projectile, rather than in a purely "lets get this thing going really fast so it does damage". Also we are dealing with a shotgun so a typical 12 bore would have a diameter of 18.5mm $\endgroup$ – Whinja Sep 9 '16 at 18:05
  • $\begingroup$ @Lord Dust A sphere of 18.5mm diameter would have a mass of just over 3 grams if made from water ice, and with suitable shaping a projectile could be expected to be in the region of 4 grams, which is in the same ballpark as a 5.56 round. $\endgroup$ – Whinja Sep 9 '16 at 18:22
  • $\begingroup$ @JDługosz the Mythbusters myth was busted on the basis that the bullet exploded on firing, this proposed solution prevents that. $\endgroup$ – Whinja Sep 9 '16 at 18:23
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You could make your projectile in a mold using lime and h2o. It would be heavy and hard enough for good penetration but would deform and dissolve enough to mask and cause confusion about the ballistic properties.

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  • $\begingroup$ Welcome to Worldbuilding, Hawk, this will leave traces of lime at the point of impact. This is hardly disappearing. Otherwise this is a good attempt at answering the question. You may have better alternatives in making a disappearing bullet work. Don't be discouraged, and have fun here! $\endgroup$ – a4android Jul 18 '17 at 5:55
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You shave a round from a cube of dry ice. This was done by a New York shooter that was eventually caught because somebody saw him shooting the rifle. He had a suppressor and custom rounds made from dry ice. The round would break down and effectively "vanish" afterwards. I remember this from when I was a child. So it was at least 12-15 years ago.

I briefly looked for the story, but could not get past other shootings. I did find a book The serial killer with disappearing bullet by Harvey Gladhill it appears to be published in 2004 so maybe my eavesdropping was about this book. Not sure.

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  • $\begingroup$ This would be a better answer if you looked up the name of the shooter and provided more details about how it worked. As is, this basically just says that dry ice (solid CO2) would work. We can guess that this is because dry ice sublimates directly to a gas under a human friendly environment. But frankly, those are the kind of details that you should be telling us. Then even people who don't know that off-hand can benefit from the answer. $\endgroup$ – Brythan Jul 19 '17 at 20:58
  • $\begingroup$ A youtube video of a shotgun shell made of dry ice youtube.com/watch?v=zVttRFI8zUo $\endgroup$ – Thorne Jul 20 '17 at 6:02
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Frozen mercury. Its density is not negligible (it's a metal, after all), it is relatively easy to obtain, although the melted remains would be easily identified in the wound, no barrel traces would be present, obviously.

The logistics of the crime would be immensely helped if the shooting occurs in winter, during a particularly cold day.

Credit for the idea goes to Jules Verne.

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    $\begingroup$ A pool of mercury would be left behind. You would get nothing about the shape of the bullet but you would know what was done. $\endgroup$ – Loren Pechtel Sep 11 '16 at 0:47

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