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Is a gun which produces no sounds within the human range of hearing possible? I know of a couple issues:

  1. Most bullets move quick enough to make a whistling sound, but I may be incorrect in that regard.
  2. The bullet may grind against the barrel of the gun, creating a grinding sound one could hear.
  3. The firing mechanism would most likely produce sound. Even something that used magnets to propel the bullet would have something keeping the bullet back, and that may grind against other metal in the gun.

Constraints:

Any sound produced must not be within the range of human hearing (20Hz to 20kHz); sound produced outside of that range is fine. The measuring device we'll use is a human hearing the firing of the gun.

This should be a reliable weapon which can reliably kill a human at 200 meters, regardless of whatever is shot or projected. It should be able to survive more than one shot, at a minimum firing rate of three times per minute.

It should be silent within Earth's atmosphere, regardless of the weather.

The technology of the time is similar to the present time, but there are advances in magnetic technology (they have far more powerful magnets as they have super compressed magnetic material and have aligned it in the same direction. All magnets used are functionally 5 times stronger, a strong dose of handwavium included).

OneSurvivor
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    Comments are not for extended discussion; this conversation has been moved to chat. – Tim B Apr 26 '18 at 12:03
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    I am not sure about worldbuilding.SE rules, but in general it's frowned upon editing a question in such a way that it invalidates current answers. Especially when it looks like a question was edited explicitly to invalidate an answer. It's better to ask a second new question instead. – David Mulder Apr 26 '18 at 13:23
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    any portable collection of forces that can propel a lethal object 200 meters will make a lot of noise. – John Apr 26 '18 at 16:26
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    Any gun, in space – Ummdustry Apr 26 '18 at 16:55
  • @DavidMulder, I suppose I can try soften the edit against the answer in question. – OneSurvivor Apr 26 '18 at 17:12
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    If such a gun fell over in the forest and nobody was there to hear it... – David Richerby Apr 26 '18 at 17:53
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    @DavidRicherby ...does it really make a sound? – OneSurvivor Apr 26 '18 at 17:57
  • There was a gun used in ww2 called the welrod that was nearly silent for its first shot. Ian of forgotten weapons on youtube had a very informative video about it. Not exactly what you're looking for, but certainly relevant. – Jonathan Apr 26 '18 at 21:29
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    Given your power requirements it can't be done. The bullet is supersonic and that produces a very, very loud crack no matter how silent the weapon. You're going to have a very hard time getting rifle-level performance at that range with anything subsonic. – Loren Pechtel Apr 26 '18 at 21:32
  • Since pretty much all the weapon design questions are on topic, I have no idea why this question was closed in the first place. – Vincent Apr 26 '18 at 22:41
  • Why do you need a .308 to reach 200m? You can reliably hit targets with lethal force at much farther with a .308, and there are smaller -- and therefore more quietable -- rounds that can reach 200m easily. Also, the bullet will make noise if you want it to hit as hard as a .308, because it'll be moving supersonically, which will create a sonic boom (barring a subsonic, extremely heavy projectile, which I'm not sure you could feasibly carry around and launch 200m). –  Apr 27 '18 at 00:47
  • @NicHartley Good point. I may include that. – OneSurvivor Apr 27 '18 at 01:09
  • Do they have room temperature superconductors? Permenant magnets are not very practical for propulsion purposes. – timuzhti Apr 27 '18 at 09:11
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    "Regardless of weather" basically rules out every gun to begin with. What if it rains? Even a subsonic bullet coming from a coil gun will inevitably be heard (and seen). The only way I see to fulfill the conditions is cheating, i.e. the gun is a mere laser pointer, and it magically summons a kobold armed with a dagger who stabs the person the laser points at. Or something. Laser guided bombs as used by the military may work too, if the sound produced by the airplane doesn't count. – Damon Apr 27 '18 at 10:13
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    I'm struggling to see how this isn't just asking for a bow and arrow. – candied_orange Apr 27 '18 at 13:06
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    @CandiedOrange Bow and arrow is not completely silent. – OneSurvivor Apr 27 '18 at 21:45
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    Arrows make a whistling noise and the head cuts the air and a humming noise as the shaft acts like a reed. – Quaternion Apr 28 '18 at 04:09
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    "Is a gun which produces no sounds within the human range of hearing possible?" In trying to answer this, the question that keeps coming up is "why"? Why absolutely quiet? "Silenced" guns aren't all that silent, but often they don't sound like a gunshot. Maybe a better question would be to outline your scenario and let people riff on a weapon to carry it out. – Schwern Apr 28 '18 at 18:18
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    What level of ambient background noise is there? Do you really need the sound pressure level to be down around 0dB? According to http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm, background in a TV studio is 20dB. Or the sound of someone breathing 1m away is given as 25dBA, although stealthy people can probably breathe more quietly than that. (Note the A-weighted dB on the 25dbA; human hearing is most sensitive around 1kHz, so it takes more sound pressure at other frequencies to sound as loud, and the threshold of hearing is higher.) – Peter Cordes Apr 29 '18 at 03:40
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    If the whistling of an arrow is too loud, than this is basically impossible without mad handwavium. – Devon M May 01 '18 at 05:20
  • @Loren Pechtel : That assumes that the projectiles emitted from the gun must be entirely passive and cannot contain any form of active propulsion or stabilization (e.g. deploying wings while in flight). Given nothing in the question specifies any requirements upon the projectiles beyond the absence of sound, why not? Moreover, there are also no stipulations against the use of adulterants of some kind (like poison) to make up for losses of raw kinetic killing power. – The_Sympathizer Dec 21 '19 at 07:22
  • It may be that the necessary engineering required here is not solely about the gun, but also the ammunition. A glider can travel much farther at a given speed than a blunt object of the same mass projected with the same speed due to its aerodynamics. And we don't need the bullet to be that good even - just not to drop too much due to gravitational pulling over the ~0.6-1 s transit time from muzzle to target (speeds 200-341 [k]m/[k]s). – The_Sympathizer Dec 21 '19 at 07:29
  • We could, perhaps, imagine an actively-stabilized (gliding), sharpened, poisoned round (think tipped with botox) launched from a coilgun suitably designed in geometry and materials to minimize its audible resonant modes as much as possible. It'd be timed to deploy its wings maybe 10-20 m from the barrel (so at about 0.05-0.1 s into the flight) and then the force from target impact breaks open the poison container, causing direct blood contact and hence blood poisoning with profound toxins. – The_Sympathizer Dec 21 '19 at 07:36
  • I'd suggest that as an answer, but am not sure enough to make definitive statements on the scientific plausibility of all those things actually having the effects I'd think they would or even if some of them could be done at all (e.g. tweaking the vibrational properties of the gun by suitable construction). – The_Sympathizer Dec 21 '19 at 07:37

15 Answers15

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It basically already exists.

Behold, the Russian S4M pistol:

PSS Silent Pistol

This firearm, like the others in its family, uses a unique piston-driven cartridge. If you look in the cutaway cartridges, you will see this piston behind the bullet. When propelled by the gunpowder charge at the rear, the piston rams the bullet forward, propelling it down the barrel.

When it reaches the end of its stroke, the piston seals on the front of the cartridge, preventing any noise or gases from escaping. The only noise in the action comes from the internal striker system, and the mechanical friction between the bullet and barrel.

As long as the bullet is subsonic (and in this design, it is) there will be no sonic boom as it passes. While there might be some noise from the striker, at most it is a 'click' rather than the loud bark of even a suppressed and subsonic conventional round.

Use an electronic primer to eliminate the noise from the striker mechanism, and integrate suppressor baffles into the barrel to eliminate any noise produced as the bullet scrapes by, and there you have it- a completely silent firearm.

Catgut
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    nice. credit images please? – Willk Apr 25 '18 at 22:13
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    The wikipedia page on that suggests that it is nearly useless outside of point-blank range, though the rifling makes the bullet look like it was fired from further away. – fyrepenguin Apr 25 '18 at 22:44
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    There's no way this thing has the muzzle velocity to make it hurt at 200m, much less compete with a .308. – chrylis -cautiouslyoptimistic- Apr 26 '18 at 02:00
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    @chrylis I was confused as to your comment, but then I saw that the question was edited to include performance characteristics after I posted this answer. This cartridge is listed as having an effective range of 25m and is capable of piercing a steel helmet at 20m. It is not a rifle round and does not on its own satisfy the updated requirements. – Catgut Apr 26 '18 at 02:12
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    Does it not make a sound when the piston bangs into the case? I imagine it must sound a little like hitting your pistol with a hammer. – Daniel Apr 26 '18 at 08:47
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    While this kind of device, especially with your proposed enhancement, is about as close as possible to completely silent, I bet it would still make some noise detectable to the human ear. – Deolater Apr 26 '18 at 12:28
  • Would be nice if you could post a video link that shows the gun in action. This would also help quell any doubts as to whether this gun is silent or not. – Nolonar Apr 26 '18 at 15:02
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    "The only noise in the action comes from the internal striker system, and the mechanical friction between the bullet and barrel." And whatever mechanism eventually stops the piston. – David Richerby Apr 26 '18 at 18:17
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    It produces sound by all 3 methods pointed out by OP. The only thing it doesn't produce is actual gunshot. – Agent_L Apr 27 '18 at 13:06
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    Youtube has a video. It's pretty quiet, still makes a "clack" though. https://www.youtube.com/watch?v=K73kKODFyzw – bobflux Apr 27 '18 at 23:15
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    If the OP really means a 0dBA sound intensity (threshold of human hearing in an anechoic chamber) for the user holding the gun, it's not plausible for anything where the bullet physically touches the barrel on the way out to meet that requirement. Interesting answer, but the OP's requirements are way too strict to allow anything like an actual gun. – Peter Cordes Apr 29 '18 at 03:44
  • THIS is what they should show in movies instead of that bullshit "phut phut" nonsense from a "silencer" attached to a conventional pistol. To get a real "silenced" firearm, it has to be built for the job. – The_Sympathizer Dec 21 '19 at 02:46
  • @Peter Cordes : Sure, but in this case, I'd think this would at least match a broader spirit of such an inquiry - being effectively what in the movies is claimed from a "silencer", such that you could shoot it in pretty much any setting but absolute or near-absolute silence, and people would not be able to locate the source of the shot by ear or even know a shot was fired apart from the wound on the target, while a suppressed firearm is still VERY audible and if you fired it in, say, an office floor, everyone would know it very clearly. – The_Sympathizer Dec 21 '19 at 02:50
  • @The_Sympathizer: This is an interesting answer to a different question. (So I guess you could call it a frame challenge). The OP seems pretty clear that they're asking for something much more extreme than a real-world version of a movie silencer, and that they realize that. e.g. they're suggesting magnetic acceleration of a bullet. – Peter Cordes Dec 21 '19 at 06:40
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No one ever mentioned a coilgun. The only noise produced would be minimal due to barrel fricion and such, because it uses magnetic fields. The biggest noise would be the capacitor discharge and the action cycling the bullet, the bonus is it has no muzzle flash. It works by wrapping a wire coil around the barrel so that when power is run through the coil it would generate a magnetic field to pull a ferromagnetic slug toward the target. Hobbyists have made some in their back yards capable of killing small game run off of 12V batteries and made of PVC pipes and magnet wire. And as a bonus you could add a destructive interference emitter.

Efialtes
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    I've never known high voltage capacitors to be anywhere close to quiet. Even camera flash capacitors are quite audible. – Deolater Apr 26 '18 at 12:29
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    @Deolater The capacitor charging frequency is selectable, make it 50kHz and you will not hear it. The discharge does not need to have a spark if you use (over) large semiconductors for all the switching. Well designed encapsulated coils would not need to have any moving parts so very low noise. With a larger weapon and longer barrel acceleration can be constrained and all forces minimised. – KalleMP Apr 26 '18 at 16:18
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    Having built these in college, I can say that the mechanism itself is relatively quiet outside of the charging of the capacitor, which, as you said, is tunable. However, if we're looking at high velocity rounds, those are not—you still have to worry about sonic booms, friction induced conflagration of the shell, and once we're past both of those, the invariable vibration of the coil. This stuff can get pretty loud. I'm not saying that your answer is necessarily wrong, but I would say it's only a step in the right direction. – Michael Macha Apr 26 '18 at 19:37
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    well i mean then you could fire golf ball rounds or just set the coil to a lower subsonic setting, i mean it's an assasination weapon not for conventional warfare. – Efialtes Apr 26 '18 at 21:17
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    Ever heard PSU coil whine? Even low powered coilgun, comparable to an airgun still produces audible crack as the coils work. – Agent_L Apr 27 '18 at 13:09
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    You don't know how much noise an electromagnet can make until you've been in an MRI machine. – Solomon Slow Apr 27 '18 at 15:30
  • @jameslarge MRI machines need to create a huge sustained magnetic field, and as such uses superconducting coils. A coil gun requires only a very brief burst of electricity. It likely would sound similar to a TMS machine (a moderate "crack" sound as the coils expand from heat). That would be hard to hear from 200 meters, especially if you use a multistage coilgun that pumps less power to each coil to achieve the same effect. – forest Apr 29 '18 at 11:16
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Laser gun

Any bullet you launch at subsonic speeds must be huge (say, an age-of-sail cannon ball) or it won't be much lethal at 200m - as in "it won't fly that long, at least in a straight trajectory". If it's supersonic, then you'll have a very audible sonic boom, no matter how hard you try. Noise cancellators are not that effective as movies would make you think (and they subtract quite a bit of power from the bullet along with some of the noise) sound wave cancellation can only be done in lab conditions.

As you have been told, laser weapons do exist, and yet again against what the movies show, the effective weapon-grade frequencies are invisible to the human eye.

nzaman
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Rekesoft
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  • I like this answer most, because I think that lasers are currently the only option for a really silent weapon. However, I fear that it doesn't fulfill the requirement of shooting a projectile. – M.Herzkamp Apr 26 '18 at 12:44
  • Sorry, the problem constraint was removed, I did not realize your answer was here before I made it. – OneSurvivor Apr 26 '18 at 17:20
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    I would argue that it's not going to be silent. Quiet maybe, but not completely inaudible: laser, especially the one powerful enough to hurt someone, will heat air, which will expand rapidly and produce noise as a result. One can hear larger camera flash crack when discharged because it emits a lot of energy in a very short burst which causes read expansion of the air, I expect the same to happen with the laser, but at larger scale. – n0rd Apr 26 '18 at 18:58
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    @n0rd You are correct a lazer capable of killing makes a lot of noise, they produce a sound similar to an arc welder... also they are very large (currently) they have an effective rage of only 2 km because lasers strong enough to punch a hole in steel also create atmospheric distortion which defocuses to beam, obviously not an issue in space... but for terrestrial use is a serious limitation. Finally if stealth is critical a large glowing beam between the firer and the target is probably not desirable. – Quaternion Apr 26 '18 at 19:13
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    I have to second that beams strong enough to actually hurt someone generally are not quiet. Look into the Mössbauer effect—that kind of force gets a lot strong when you're up above 3W. Additionally, you have light induced florescence to worry about—the creation of plasma (even a spark) by an intense beam, which is not silent.

    Though to be fair, these sounds do not make a person immediately think "gunfire".

     – Michael Macha Apr 26 '18 at 19:40
  • Perhaps the problem of heating the air could be avoided by changing the frequency of the radiation. – HRSE Apr 27 '18 at 02:32
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    High powered laser is very loud, as it heats the air on the whole path. https://www.youtube.com/watch?v=5_9ac-w4DW8 - every bang you hear is the laser turned on once. – Agent_L Apr 27 '18 at 13:13
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    Just a niggling point, modern firearm suppressors actually increase the speed of the fired round by a small percentage, so you gain downrange energy, not lose it. If you used older technology suppressors with wipes and gaskets, you'd be correct. See http://www.thetruthaboutguns.com/2012/05/foghorn/ask-foghorn-does-a-silencer-effect-the-velocity-of-the-bullet/ – delliottg Apr 27 '18 at 16:18
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    What about a laser that gradually ramps up power over, say, half a second? You'd have to keep the gun trained steadily, but since the air is heated less "instantaneously" there should be less noise. Additionally, by using a normally invisible spectrum (far IR or the like), it'd be invisible without special equipment -- which the shooter could have to aid in targeting: half-pull of the trigger for low power IR laser to target, then full pull to ramp up to full blast and do the deed. – Doktor J Apr 27 '18 at 16:52
  • A certain statistic may be of interest here: The number of humans killed by lasers since their invention: 0 – Monty Wild Jun 16 '20 at 14:00
15

Airgun with dimpled bullets.

In The Adventure of the Empty House, Colonel Adair attempts to assassinate Holmes using a special air gun. After he is captured Holmes looks over the gun.

Holmes had picked up the powerful air-gun from the floor and was examining its mechanism. “An admirable and unique weapon,” said he, “noiseless and of tremendous power. I knew Von Herder, the blind German mechanic, who constructed it to the order of the late Professor Moriarty. For years I have been aware of its existence, though I have never before had the opportunity of handling it. I commend it very specially to your attention, Lestrade, and also the bullets which fit it.”

Propelling the bullet via compressed air there is no explosion. There is, however, a "strange loud whiz" as the bullet traverses the air.

This strange loud whiz is from the air passing the bullet. This could be reduced by using a more aerodynamic bullet. Colonel Moran's gun fired unusual bullets that were thought to be soft nosed revolver bullets. Laminar flow is less noisy than turbulent flow. By smoothing airflow around the bullet you will reduce noise produced by the air. You could do this with dimples, like a golf ball.

In their pursuit of a lower-drag bullet, the Army tried a variety of designs... The dimpled “golf-ball” design was considered a “long shot” according to the design team, but it has performed beyond all expectations. The nominal drag coefficient (Cd) has improved by about +.040, while cartridge muzzle velocity has increased by nearly 80+ fps because the bullet’s dimpled skin reduces in-barrel friction. What’s more — the terminal performance of the dimpled bullet has been “spectacular”. The Aberdeen team set out to produce a slightly more slippery bullet for U.S. Army snipers. What they ended up with is a bullet with dramatically enhanced long-range ballistics and superior killing power on “soft targets”.

dimpled bullet

Fivesideddice
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Willk
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    Is the Holmes' airgun even realistic? I mean, of course, airguns exist, but what about the stopping power and range? – Gnudiff Apr 26 '18 at 04:49
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    @Gnudiff there are airguns that you can hunt big game with. – Nuloen The Seeker Apr 26 '18 at 06:51
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    The dimples do not create a laminar flow. They, and similar solutions such as vortex generators on aircraft wings, specifically create a turbulent flow in the surface layer. A turbulent flow follows the surface better which results in the surface layer being smaller and more stable. With shapes such as balls and bullets that otherwise generate a trailing vortex street this reduces overall drag since the oscillation of the vortex street wastes lots of energy. The oscillations also produce noise and reduce accuracy. The accuracy loss is probably the main reason golf balls are dimpled. – Ville Niemi Apr 26 '18 at 08:11
  • I am guessing the "laminar flow" and "dimpled bullets" should just be in separate paragraphs or otherwise clearly separated. – Ville Niemi Apr 26 '18 at 08:15
  • @Gnudiff - yup, even Lewis & Clark took a air rifle on their famous exploration trip. Check wiki/google/whatever for the Girandoni air rifle. – ivanivan Apr 26 '18 at 11:05
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    Airguns exist that are quite powerful (though probably less than the power the question now requests), but powerful airguns are quite loud. Not as loud as firearms, but not literally silent as the question requests. – Deolater Apr 26 '18 at 12:30
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    I shoot air pistol competitively and I find quite uncomfortable to do it without some sort of hearing protection. And that's PCP pistol with minimal amount of moving parts. Lever action ones are even louder. – n0rd Apr 26 '18 at 18:50
  • Airguns were actually issued as military firearms in the late 1700's, see the Girandoni air rifle: https://infogalactic.com/info/Girandoni_air_rifle – Thucydides Apr 27 '18 at 05:05
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    You do know that the cited dimpled bullet article was an April 1st spoof, right? – DJohnM Apr 28 '18 at 06:37
  • @DJohnM yeah, of course. Sure I knew that, or something. – Willk Apr 30 '18 at 02:51
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There are already a wide variety of supressed weapons out there. Russian "Captive Piston" rounds are likely the best way to use conventional weapons (simply load 9mm captive piston bullets in a Glock, for example), so long as you make allowances for the reduced range and penetration compared to normal rounds. This isn't limited to pistols, the US Navy developed a captive piston 12 gauge shotgun shell during the Viet Nam war. The US Army also experimented with captive piston and other silenced weapons for "tunnel rats".

enter image description here

12 gauge "telecartridge"

Perhaps the best place to look for inspiration would be some of the assassination weapons designed in the Second World War. In addition to subsonic rounds, many used locked mechanisms so there would be no noise as the action cycled, or extreme versions of suppressors to capture and muffle the sounds of expanding gasses.

enter image description here

Welrod Mk 1 pistol

The De Lisle carbine is perhaps the most extreme example from that period, essentially turning the entire barrel into part of the suppressor:

The Thompson gun barrel was ported (i.e. drilled with holes) to provide a controlled release of high pressure gas into the suppressor that surrounds it before the bullet leaves the barrel. The suppressor, 2 inches (5.1 cm) in diameter, went all the way from the back of the barrel to well beyond the muzzle, making up half the overall length of the weapon. The suppressor provided a very large volume to contain the gases produced by firing; this was one of the keys to its effectiveness.

This was far superior to the more complex suppressor of the Welrod pistol. As an aside, the carbine allows it to have an effective range of 200m, and while being shot with a.45 ACP isn't as powerful as a .308, it most certainly does the job (silent sentry takeouts, for example).

enter image description here De Lisle carabine

So the short answer is to use captive piston type ammunition if you want to use conventional firearms, or weapons with subsonic ammunition and extreme suppressors if you are willing to use specialized weapons.

Thucydides
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    None of what you wrote is going to meet his "This should be a reliable weapon which can kill a human at 200m, with similar force and killing power as a .308 rifle" constraint. (Not that anything will...) – RonJohn Apr 25 '18 at 21:57
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    Well the "science based" tag does have some constraints...... – Thucydides Apr 26 '18 at 01:23
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    And OP is asking the impossible... – RonJohn Apr 26 '18 at 03:34
  • I have shot a replica de lisle and can confirm it is bonkers quiet. We were in an indoor 25m range and the only audible sound was the clang as the 45acp round slammed into the metal backstop. 200m would be a stretch though maybe not impossible, I think we used it at 100 a couple of times. – Joseph Rogers Apr 26 '18 at 17:05
  • Owning both a 9mm carbine and a 45acp carbine, hitting something at 200 yards is almost an artillery exercise. Lots of fun though. – ivanivan Apr 26 '18 at 17:54
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    The De Lisle carbine is actually supposed to be able to hit targets at up to 400m, with 200m being it's effective range. More power to those Commandos and SOE operators in WWII, and British SAS and SBS troops who supposedly continued to use it in Malaysia, Northern Ireland, Yemen and even into the Falklands Islands War. – Thucydides Apr 26 '18 at 22:15
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A "cold-load" round is subsonic meaning the bullet doesn't make much sound in transit, nor does the escaping gas behind it. Electronic firing, and a smooth bore could do away with all mechanical sound but the recoil is still going to make a "sound" travelling through the body of the person firing the gun.

The only completely silent weapon I can think of is one that produces exactly the same set of vibrations with every shot, from projectile and weapon, and then uses destructive interference to cancel out that vibration perfectly at source.

Ash
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    I like the destructive interference idea the most so far. – OneSurvivor Apr 25 '18 at 19:51
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    A more relatable form of destructive interference would be active noise cancelling headphones. It's theoretically possible to cancel a sound perfectly, but not generating it from exactly the same point/surface will create pockets of audibility radiating away from the source. – Samuel Apr 25 '18 at 20:15
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    Where did you get the idea that electronic firing is illegal? The Wikipedia article links to such a civilian product. It was commercially unsuccessful. – user71659 Apr 25 '18 at 21:16
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    Electronic firing is fine, what you do with it is another matter. What's critical is one trigger pull, one bullet and no easy way to change that. Also, if the bullet isn't going to fire immediately you have to keep the trigger pulled until it either fires or you change your mind. – Loren Pechtel Apr 26 '18 at 21:38
4

You won't totally eliminate all noise - but you can get really really close, so that any noise made is less or equal to ambient noise in a forest, etc.

Start with a bolt action or other manually operated rifle action.

Then, get something in the right caliber. You want 308/762x51 performance, it ain't gonna happen. No way to get something moving to 2700fps without breaking sound barrier.

What will work ballistics wise AND noise control wise would be something like the 300 Blackout - or even properly loaded 308/762x51, although the 300BO will be much easier to develop a "vewy vewy quiet for huntin' wabbits" load.

You are going to want to get a 200-230 grain bullet (15.4gr per gram if you are wondering) moving at 1050fps velocity at the muzzle, with as little gas volume generated as possible.

Then you need a proper suppressor, designed around the bullet diameter AND anticipated gas volume being generated that needs to be controlled.

You can build something like this TODAY for about $2500 plus tax stamp(s) and FFL fees if you live in the US and in a state that allows SBRs (short barrel rifle) and suppressors. I've shot and (not?) heard shot several AR15 builds in 300bo where all the shooter hears is the sound of the action working, the click of the hammer/firing pin impacts, and a "pffft" like opening a can of soda. A dB meter phone app measured it at 76dB about 10 feet away from the shooter - with a manually operated rifle, it would be quieter until you cycled the action. Carries similar energy at 200 yards as a 45acp does at the muzzle - 220-240grn bullet moving at 800fps. Accurate enough to hit clay pigeons (4.5" disc) at 200 yards.

A 308 bolt gun, with subsonic 220 grn bullets, still let out a rather large "sigh" - still hearing safe, but the guy using it said that it would spook animals that were within 25 yards or so (contract hunter for feral pig control in some local wildlife preserves...).

Edit - just came back from local shooting range, there happened to be a guy there with a suppressed 300BO bolt action. His suppressor is home made and needs improvement (but that requires a new tax stamp) but from 10 feet away with no hearing protection in I could hear the whack of the firing pin and the pfft of gas. Tried using a dB meter on my phone but the noise from adjacent ranges (separated by 15' earth berms) was masking the local noise....

ivanivan
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    +1 for an actual dB measurement, and attempted on another gun, but note that the OP complained that a bow and arrow isn't completely silent for the person using it. Without further clarification, I think we need to assume 0dBA, the threshold of human hearing with no background noise, which is obviously ridiculous and unanswerable with any fast-moving parts. – Peter Cordes May 01 '18 at 09:47
  • @PeterCordes Yup. Could maybe get closer - bigger/better suppressor, electronic ignition to avoid the whack of the hammer on the firing pin, etc. OP should be looking at a laser or similar truly silent weapon. – ivanivan May 01 '18 at 13:52
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    Good luck building a 0dBA laser with killing power; see other comments: it'll heat the air enough to make some sound! – Peter Cordes May 01 '18 at 13:55
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A well-suppressed firearm using subsonic ammunition is about as quiet as you can get with a kinetic projectile. Virtually all of the sound of the powder combustion and bullet acceleration is contained in the barrel/chamber lock-up and all of the gas ejecting is contained by baffles in the suppressor (the larger/longer the suppressor, the more is suppresses). The bullet, being subsonic, won't make a "crack". Of course a slower bullet is less accurate as it still must drop according to gravity (and some things like spin stabilized drag may not be as effective) and will hit with FAR less energy than a supersonic projectile, so either you have to be really close to the target or use a big heavy projectile that will rely on mass to maintain sufficient impact energy (or have a projectile that relies on some other mechanism to incapacitate/kill, like poison, explosives, etc).

So now the only sound is the actual gun mechanism operating, i.e. hammer hitting a firing pin, or ejecting one round and loading another. This can be eliminated via a striker firing mechanism or even an electric one with few to no moving parts. The action of the firearm can be single shot (bolt action or break open) or at least be locked into single shot when needed (the Navy SEAL "hush puppy" 9mm S&W Mark22 pistol does this to prevent ejecting the cartridge which produces sound and a visual indicator) or/in addition to using something like caseless ammo or even a "metal storm" type preloaded barrel filled with bullets that are ignited in sequence (front to back) to reduce almost all moving parts. Metal storm weapons have variable accuracy (each round has increasing barrel length to build up velocity and thus have a higher point of impact) and require a barrel change to reload, but with a proper suppressor they could be virtually silent.

A revolver can also be suppressed, but they require a tighter lockup between the rotating cylinder and the barrel to prevent sound from that area, or need a bulky shroud around it for maximum noise suppression. More info here

Jason K
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    Suppression does not make the gun silent, or even remotely close to it. It's still extremely loud if only because of the detonation itself. Silencers are designed only to reduce the sound to levels that do not cause permanent hearing loss, not to make the gun silent. – forest Apr 29 '18 at 11:26
  • @forest It all really depends on your specifications. You can wrap a metal storm pre-loaded tube in noise dampening foam and give it a 2 foot long, 8 inch wide suppressor at the muzzle end to contain the gasses (far larger than most suppressors) and you can make it extremely quiet. It all depends on low large and bulky you make it versus how quiet you need it to be. – Jason K May 07 '18 at 14:02
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Magnetic dart gun. Use magnetic rings to pull the dart to speed. No other barrel exists except the magnets so that air pressure can't build up anywhere. The dart is probably more like a long thin bullet.

Since the pressures on the "Bullet" are less jarring and better distributed than a gunpowder accelerated round, the bullet could more easily be made to deform/shatter on impact so that it wouldn't just slide through the body cleanly.

The bullet could be any length--from a sewing needle up to the length of a knitting needle--with tiny dart-like fins at the back to keep it pointed in the right direction (they could also impart spin). As long as it shattered on impact and had some weight it would be terribly deadly.

If a slower speed (Subsonic) were desired, it could have active guidance allowing it to be fired on an arc. A long thin slow sharp bullet could penetrate the body and shatter or even explode inside using the body's mass as a muffler...

Bill K
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In another question, we were asked about the feasibility of a weapon that would fire ammunition propelled at near-light-speeds. Even a 9mm bullet fired at such relativistics would never be heard by its victims, nor the shooter, nor anyone within a few kilometers from the shot. Everybody would be vaporized before any sound could be produced. The blast will be audible to those who are not caught in the fireball/mushroom cloud, though.

Another option would be to couple any regular gun to an LRAD weapon. Set the LRAD to audible frequencies, and trigger it before the regular gun's shots. Anyone in the cone of action of the LRAD won't be able to hear the shots. They probably won't see it too, because they might be in fetal position with their eyes closed, and if they can stand, they would be puking.

Barring those options, you could, you know, use small pistols with supressors? As long as you are far away from any targets the loudest sound will be the bullet impacts.

If you really need to go for no sound at all: tazers, lasers, LRAD's can be silent. I have fired with some bows as well and they are much less noisy than guns.

The Square-Cube Law
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    @Samuel if you check my answer in the link, you'll see that according to Munroe, a potential shooter of such a weapon would already be disintegrating within the first 30 nanoseconds of having fired the gun. The fireball spreads faster than sound, and for the highest pitches we can hear (around 20 Khz), the interval between two sound waves is enough to vaporize a person 1600 times. Air will eventually vibrate into a very loud boom, but that will be after everybody in the blast radius has been disintegrated. – The Square-Cube Law Apr 25 '18 at 20:26
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    @Samuel from the relevant XKCD what if article: "The ball is going so fast that everything else is practically stationary. Even the molecules in the air are stationary. Air molecules vibrate back and forth at a few hundred miles per hour, but the ball is moving through them at 600 million miles per hour. This means that as far as the ball is concerned, they’re just hanging there, frozen." – The Square-Cube Law Apr 25 '18 at 20:29
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    I have reworded that part for clarity. – The Square-Cube Law Apr 25 '18 at 20:32
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    If you want a bullet which is not heard by the victim before it strikes, just use regular supersonic bullets. – vsz Apr 26 '18 at 04:09
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How about a silencer that uses active noise cancellation? It wouldn't have to be on the barrel - actually, it wouldn't have to be on the gun at all, but probably nearby. You may have to program it for a specific weapon, but in theory, it could cover the blast of several nearby weapons at once.

Shawn V. Wilson
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    No it could not, or it could but only for a single observer. – Jasen Apr 26 '18 at 04:49
  • @Jasen - If the round is subsonic, then such a noise canceller is possible - more or less. The report (as opposed to wake noise) is emitted from a very small, point-like source - the exit aperture of the barrel. Such a noise source is eminently addressable by a noise cancellation mechanism. – WhatRoughBeast Apr 26 '18 at 12:59
  • yeah, but only one located at the muzzle. may as well use a supressor – Jasen Apr 26 '18 at 23:39
  • Or use both a suppressor and the noise cancellation. – Jiminion Apr 27 '18 at 14:13
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It should be possible to create a'gun' that accelerates the projectile with a magnetic field. This would also levitate the projectile. No noise from mechanical contact or explosive. The longer the barrel the better. A slow acceleration would seem better for not producing an audible shock wave.

stu
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  • Welcome to Worldbuilding.SE. When you get a moment, please take our [tour] and visit our [help] to learn more about us. This is an OK first answer, but bear in mind the OP is asking for no sound at all... including the passage of the bullet through the atmosphere. Rail guns only remove the "bang" of black powder. What would you do to the round to reduce the sound of it's passage? – JBH Apr 26 '18 at 08:23
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    If you can use a coilgun to accelerate a round to supersonic velocities, in principle you could produce a sub-sonic coilgun with a reasonably heavy round. That would certainly be lethal while still remaining entirely silent apart from say..capacitor whine. – Ruadhan Apr 26 '18 at 14:57
  • Make the projectile a circular bullet with a hole through the middle (like a tall, thin doughnut), using the principle of a Busemann biplane to minimise the sonic boom, while keeping the projectile supersonic? – Chronocidal Jun 16 '20 at 07:43
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In case of surviving in hostile world and having only some basic stuff, a slingshot might be an option (as well as bow/arrows, but they were already mentioned above).

Vitalii Vasylenko
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    @RickM.Thanks! Well, you just take a bigger slingshot - that's it :) If to talk seriously, in case of surviving in hostile world and having only some basic stuff, a slingshot might be an option (as well as bow/arrows, but they were already mentioned above). – Vitalii Vasylenko Apr 29 '18 at 20:33
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    @RickM. Sure, done. – Vitalii Vasylenko Apr 29 '18 at 21:46
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Perhaps a gamma ray beam instead of a standard laser gun? It is outside of human sight and would very very likely give someone sever radiation damage and/or terminal cancer depending on how long you bombard them. Plus, completely silent. This is silent as the source of the radiation would be some radioactive material rather than any sort of electronically generated force or energy. Just make sure you have sufficient shielding. (https://physics.aps.org/articles/v9/50) Edit: Note that the ability to form the gamma radiation into a laser instead of a beam is very much desired in physics, but as of the date of this post is scientifically un-achieved. en.wikipedia.org/wiki/Gamma-ray_laser

Erick Stone
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  • @Erick_Stone , Is there a way to focus the beam, so that it won't spread in every direction? Also, how much power does this need? A lot of power generation tends to be loud. – Rick M. Apr 29 '18 at 20:51
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    @RickM. https://en.wikipedia.org/wiki/Gamma-ray_laser An actual singularized beam is a much sought after physics desire unfortunately. In this case, the power involved is not a generated power, but a radioactive power. You would need some kind of radioactive material as the generator which would be silent. – Erick Stone Apr 30 '18 at 18:57
  • Interesting! When you got a moment, can you edit that info into the answer to make it more complete? – Rick M. May 01 '18 at 15:16
  • Most speculative articles about gamma ray lasers suggest you need a nuclear device to provide the starting energy. The gamma ray laser will also make a "crack" noise as it passes through and ionizes the air, but you might lose that in the explosion of the driving device....... – Thucydides May 01 '18 at 16:17
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I can attest from personal experience that a .22 rimfire rifle loaded with subsonic .22 short ammunition and fired through a flywire silencer can be completely silent to all intents and purposes except for the "clink" of the firing pin against the cartridge, and the sound of the impact of the bullet against the target.

Silent for all intents and purposes means that the sound produced is below that of ambient noise and/or below the threshold of hearing.

If we were to substitute an electrical firing mechanism for the rimfire primer, the cartridge's contents might be ignited effectively silently.

This would allow a subsonic bullet to be fired effectively silently. We may discount the sound of projectile impact

Now, it may be argued that a .22 is of limited lethality, however there are options that can improve that. By using a .22 hollow-point bullet, the hollow point may be filled with a toxic substance... Perhaps curare. A curare-tipped .22 bullet, even if fired subsonically, could inflict enough damage to penetrate the skin at 200m, and with a sufficiency of curare, even an otherwise minor injury could easily be fatal as a result of the poison. The tip of a hollow point round could easily hold enough curare in pure form to kill most humans.

Monty Wild
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