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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).

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    $\begingroup$ 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. $\endgroup$
    – David Mulder
    Commented Apr 26, 2018 at 13:23
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    $\begingroup$ If such a gun fell over in the forest and nobody was there to hear it... $\endgroup$
    – David Richerby
    Commented Apr 26, 2018 at 17:53
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    $\begingroup$ 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. $\endgroup$
    – Loren Pechtel
    Commented Apr 26, 2018 at 21:32
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    $\begingroup$ I'm struggling to see how this isn't just asking for a bow and arrow. $\endgroup$
    – candied_orange
    Commented Apr 27, 2018 at 13:06
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    $\begingroup$ "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. $\endgroup$
    – Schwern
    Commented Apr 28, 2018 at 18:18

15 Answers 15

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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.

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    $\begingroup$ nice. credit images please? $\endgroup$
    – Willk
    Commented Apr 25, 2018 at 22:13
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    $\begingroup$ 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. $\endgroup$
    – fyrepenguin
    Commented Apr 25, 2018 at 22:44
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    $\begingroup$ There's no way this thing has the muzzle velocity to make it hurt at 200m, much less compete with a .308. $\endgroup$
    – chrylis -cautiouslyoptimistic-
    Commented Apr 26, 2018 at 2:00
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    $\begingroup$ @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. $\endgroup$
    – Catgut
    Commented Apr 26, 2018 at 2:12
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    $\begingroup$ 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. $\endgroup$
    – Daniel
    Commented Apr 26, 2018 at 8:47
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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.

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    $\begingroup$ I've never known high voltage capacitors to be anywhere close to quiet. Even camera flash capacitors are quite audible. $\endgroup$
    – Deolater
    Commented Apr 26, 2018 at 12:29
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    $\begingroup$ @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. $\endgroup$
    – KalleMP
    Commented Apr 26, 2018 at 16:18
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    $\begingroup$ 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. $\endgroup$
    – Michael Macha
    Commented Apr 26, 2018 at 19:37
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    $\begingroup$ 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. $\endgroup$
    – Efialtes
    Commented Apr 26, 2018 at 21:17
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    $\begingroup$ Ever heard PSU coil whine? Even low powered coilgun, comparable to an airgun still produces audible crack as the coils work. $\endgroup$
    – Agent_L
    Commented Apr 27, 2018 at 13:09
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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.

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    $\begingroup$ 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. $\endgroup$
    – n0rd
    Commented Apr 26, 2018 at 18:58
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    $\begingroup$ @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. $\endgroup$
    – Quaternion
    Commented Apr 26, 2018 at 19:13
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    $\begingroup$ 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". $\endgroup$
    – Michael Macha
    Commented Apr 26, 2018 at 19:40
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    $\begingroup$ High powered laser is very loud, as it heats the air on the whole path. youtube.com/watch?v=5_9ac-w4DW8 - every bang you hear is the laser turned on once. $\endgroup$
    – Agent_L
    Commented Apr 27, 2018 at 13:13
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    $\begingroup$ 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 thetruthaboutguns.com/2012/05/foghorn/… $\endgroup$
    – delliottg
    Commented Apr 27, 2018 at 16:18
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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

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    $\begingroup$ Is the Holmes' airgun even realistic? I mean, of course, airguns exist, but what about the stopping power and range? $\endgroup$
    – Gnudiff
    Commented Apr 26, 2018 at 4:49
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    $\begingroup$ 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. $\endgroup$
    – Ville Niemi
    Commented Apr 26, 2018 at 8:11
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    $\begingroup$ 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. $\endgroup$
    – Deolater
    Commented Apr 26, 2018 at 12:30
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    $\begingroup$ 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. $\endgroup$
    – n0rd
    Commented Apr 26, 2018 at 18:50
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    $\begingroup$ You do know that the cited dimpled bullet article was an April 1st spoof, right? $\endgroup$
    – DJohnM
    Commented Apr 28, 2018 at 6:37
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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.

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    $\begingroup$ 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...) $\endgroup$
    – RonJohn
    Commented Apr 25, 2018 at 21:57
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    $\begingroup$ Well the "science based" tag does have some constraints...... $\endgroup$
    – Thucydides
    Commented Apr 26, 2018 at 1:23
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    $\begingroup$ And OP is asking the impossible... $\endgroup$
    – RonJohn
    Commented Apr 26, 2018 at 3:34
  • $\begingroup$ 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. $\endgroup$
    – Joseph Rogers
    Commented Apr 26, 2018 at 17:05
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    $\begingroup$ 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. $\endgroup$
    – Thucydides
    Commented Apr 26, 2018 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.

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    $\begingroup$ I like the destructive interference idea the most so far. $\endgroup$
    – OneSurvivor
    Commented Apr 25, 2018 at 19:51
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    $\begingroup$ 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. $\endgroup$
    – Samuel
    Commented Apr 25, 2018 at 20:15
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    $\begingroup$ Where did you get the idea that electronic firing is illegal? The Wikipedia article links to such a civilian product. It was commercially unsuccessful. $\endgroup$
    – user71659
    Commented Apr 25, 2018 at 21:16
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    $\begingroup$ 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. $\endgroup$
    – Loren Pechtel
    Commented Apr 26, 2018 at 21:38
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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....

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    $\begingroup$ +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. $\endgroup$
    – Peter Cordes
    Commented May 1, 2018 at 9:47
  • $\begingroup$ @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. $\endgroup$
    – ivanivan
    Commented May 1, 2018 at 13:52
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    $\begingroup$ Good luck building a 0dBA laser with killing power; see other comments: it'll heat the air enough to make some sound! $\endgroup$
    – Peter Cordes
    Commented May 1, 2018 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

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    $\begingroup$ 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. $\endgroup$
    – forest
    Commented Apr 29, 2018 at 11:26
  • $\begingroup$ @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. $\endgroup$
    – Jason K
    Commented May 7, 2018 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...

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  • $\begingroup$ Hmmm, this might actually work. $\endgroup$
    – OneSurvivor
    Commented May 3, 2018 at 19:08
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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.

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    $\begingroup$ @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. $\endgroup$
    – The Square-Cube Law
    Commented Apr 25, 2018 at 20:26
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    $\begingroup$ @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." $\endgroup$
    – The Square-Cube Law
    Commented Apr 25, 2018 at 20:29
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    $\begingroup$ I have reworded that part for clarity. $\endgroup$
    – The Square-Cube Law
    Commented Apr 25, 2018 at 20:32
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    $\begingroup$ If you want a bullet which is not heard by the victim before it strikes, just use regular supersonic bullets. $\endgroup$
    – vsz
    Commented Apr 26, 2018 at 4:09
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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.

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    $\begingroup$ No it could not, or it could but only for a single observer. $\endgroup$
    – Jasen
    Commented Apr 26, 2018 at 4:49
  • $\begingroup$ @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. $\endgroup$
    – WhatRoughBeast
    Commented Apr 26, 2018 at 12:59
  • $\begingroup$ yeah, but only one located at the muzzle. may as well use a supressor $\endgroup$
    – Jasen
    Commented Apr 26, 2018 at 23:39
  • $\begingroup$ Or use both a suppressor and the noise cancellation. $\endgroup$
    – Jiminion
    Commented Apr 27, 2018 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.

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  • $\begingroup$ Welcome to Worldbuilding.SE. When you get a moment, please take our tour and visit our help center 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? $\endgroup$
    – JBH
    Commented Apr 26, 2018 at 8:23
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    $\begingroup$ 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. $\endgroup$
    – Ruadhan
    Commented Apr 26, 2018 at 14:57
  • $\begingroup$ 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? $\endgroup$
    – Chronocidal
    Commented Jun 16, 2020 at 7:43
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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).

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    $\begingroup$ @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). $\endgroup$
    – Vitalii Vasylenko
    Commented Apr 29, 2018 at 20:33
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    $\begingroup$ @RickM. Sure, done. $\endgroup$
    – Vitalii Vasylenko
    Commented Apr 29, 2018 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

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  • $\begingroup$ @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. $\endgroup$
    – Rick M.
    Commented Apr 29, 2018 at 20:51
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    $\begingroup$ @RickM. 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. $\endgroup$
    – Erick Stone
    Commented Apr 30, 2018 at 18:57
  • $\begingroup$ Interesting! When you got a moment, can you edit that info into the answer to make it more complete? $\endgroup$
    – Rick M.
    Commented May 1, 2018 at 15:16
  • $\begingroup$ 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....... $\endgroup$
    – Thucydides
    Commented May 1, 2018 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.

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