# Are bullets slower than sound feasible in a high tech scenario?

Is it possible to have futuristic bullets that travel slower than sound? I'm afraid they would not carry as much kinetic energy as the supersonic variants. I know we have subsonic ammunition as of today, but in the future as armor technologies advance, the bullets will need more power.

Is it a total no-no to have subsonic bullets in a gritty sci-fi scenario, or is there a way to make them believable?

Notice the question is not tagged , just .

• Don't target the armor... – AlexP Apr 11 '17 at 13:40
• We live in a high tech society and there are plenty of subsonic ammo. The speed of sound is 1126 ft/s. Here is a chart: mcarbo.com/22LR-Ballistics-for-your-Rifle.jpg Maybe if everyone wears armor, these can be non-lethal rounds. Or maybe the weight of the bullet is so great there is still lethal kinetic energy (cannon ball). – Chloe Apr 11 '17 at 17:05
• How tied to the traditional Gun -> Bullet definition? With modern technology getting to were it is, one can see the case of the micro Hellfire Missile traveling at sub-sonic speed. – Enigma Maitreya Apr 11 '17 at 19:54
• Crossbows still exist today.. and seem relatively effective. – Mikey Apr 12 '17 at 1:21
• Dune (Frank Herbert)'s armors could only be penetrated by slow-moving objects (such as knifes). This concept can de adapted. – Guilherme Bernal Apr 12 '17 at 10:45

If you don't want to slow down your bullet, increase the speed of sound !

In a pure sci-fi scenario you may make your bullet from a metal/composite/electronic device/... that heat up the air around it. Because the warmer the air is the higher is the speed of sound. You can alternatively burst a high energy infra-red ray to warm up the air just before shooting your bullet. By doing so the bullet will travel in an "air highway" and won't reach the speed of sound in that tube, and it will have a high speed kinetic energy !

For the math let us say your bullet travel at 900 m/s (muzzle velocity of a medium range weapon) we have : $$c_{air} = 331.3 * \sqrt{1 + \frac{T°C}{273.15}}$$ thus, $$T°C = (\frac{c_{air}^2}{331.3^2} - 1)*273.15$$ for a sound speed around 1000m/s (100m/s higher than the bullet) it would need air at $2215°C$ ! You don't want to put you eyebrows next to hit...

Note : Warming the air so quickly at such a temperature would probably create a deflagration with a noise (and a force) bigger than your bullet. But still, the bullet is slower than the speed of sound is it ?

• you win the most out-of-box answer award... – Mindwin Apr 11 '17 at 17:27
• Any directed beam used to heat the air would go in a straight line. Bullets follow a ballistic arc(en.wikipedia.org/wiki/Trajectory_of_a_projectile). Therefore, the path the bullet follows would not be the same as the beam. – Andrew Neely Apr 11 '17 at 19:31
• "Someone's shining a hot light on me - I should move" says the target. – Criggie Apr 12 '17 at 8:40
• if you are heating the air to 2215 why would you bother with the bullet? – jk. Apr 12 '17 at 10:57
• Also, if we heat the air, the air, now being lighter, would rise out of the bullet's path. – Andrew Neely Apr 12 '17 at 18:21

Subsonic rifle rounds can go through modern body armor. Using new harder materials on the bullets I don't see this being less true in the near future.

Also penetration isn't required. Given enough blunt force trauma you can still kill your target. Modern technology might even aid in aiming. Making headshots easier. We still can't make good bullet proof helmets I think.

• it's not about the helmet, but about the neck of the person. The head is far less massive than the abdomen, therefore the momentum transferred while impacting the helmet deals a bigger delta v, which neck muscles cannot normally withstand. – L.Dutch - Reinstate Monica Apr 11 '17 at 14:04
• @Luaan Some military body armor is sort of brittle, and yeah, multiple hits degrade the armor, but you're still going to have better effect with higher-energy projectiles. If you can blast through the armor eventually using subsonics, you'll do much better with more kinetic energy. This is at best an argument for subsonics being better than nothing, not better than supers. – Deolater Apr 11 '17 at 14:56
• @Deolater: The 9x39 SP6 is specifically designed to take out military grade bodyarmor with a suppressed weapon. – Mormacil Apr 11 '17 at 14:57
• Also, a big advantage of subsonic rounds is that they're substantially quieter and easier to suppress than supersonic rounds. It's reasonable to assume that stealth can still be important (or even preferred) in "high tech" scenarios. – aroth Apr 12 '17 at 3:59
• It's worth noting that body armor is not designed to prevent you from getting hurt, it's designed to prevent you from getting killed. – T.J.L. Apr 12 '17 at 13:01

For (most) bullets to do damage it needs kinetic energy. At its basics it's $\frac{1}{2} \times mass \times velocity^2$. To have a small bullet to do large amounts of damage you give it a lot of speed. So much so they go supersonic.

So your first option is to increase the size of the bullet = more mass = more damage done on the receiving end. Use something like a multi stage propellent and the kickback might be manageable. Also known as hand cannons.

The second option is to increase the density of the bullet. Kind of like the A-10's ammo.

Third option is to leave the simple slug throwers in the past and not depend on the transferred energy to kill. So go with lower bullet speed and let the bullets other properties do the work. Explosions are always nice. But attract a bit of attention. And if you want to kill the bang of the gun, I think you don't like explosions that much. If your miniaturisation is very good, go with shaped charge. A nice jet of plasma cuts quite well.

*edit, I have been notified in the comments (thanks to Pokechu22) that:

Kinetic energy is (1/2) * mass * velocity^2; it's momentum that's just mass * velocity. Same idea, just slightly different (and the squared means that an increase in velocity is more important than an increase in mass)

• Fourth option - Make the tip of the bullet dense, hard, and sharp. That will give better penetration against most armor types, although also result in less energy transfer to the target (as the bullet will tend to pass straight through something squishy like a person and then come out the other side rather than coming to a complete stop within the target). Kinetic energy only counts when it's delivered to the target and not expended elsewhere. – aroth Apr 12 '17 at 4:02
• Coat the bullet w/ poison. It just has to touch his skin. (yeah, I know, poison is a woman's murder weapon, blah blah Joffrey) – Carl Witthoft Apr 12 '17 at 17:51
• While poison sounds nice, it need to be injected mostly. Or else it can become a danger to the shooter himself. So it can be stopped by thick clothing or something that breaks the needle. At the same time, poisons need time to work, minutes to hours at the minimum. The nice part about bullets is that they can kill very speedily. – Flummox - don't be evil SE Apr 12 '17 at 18:44
• +1 for your third option. Depending on what the OP means by "bullet", in terms of size, an explosive option makes a lot of sense. A subsonic "bullet" with a suppressed gun would make it harder to locate the source, while the "bang" at the target wouldn't reveal much except that an attack was taking place -- something you might figure out anyhow when the target drops. – Wayne Apr 12 '17 at 20:03

In Seveneves Neal Stephenson describes weapons that fling tiny robots as bullets. Specifically designed not to penetrate the hull for a spacecraft, but carried a whole host of nasty tools to hurt targets. They could differentiate between targets and even change direction in the air.

• I'm not really sure that referencing a "speculative fiction novel" meets the criteria for [science-based]. Can you flesh this out a bit? – user Apr 12 '17 at 8:36
• @MichaelKjörling What more is there to flesh out? I've stated the source and summarized the concept. The book is speculative fiction but more specifically it's science fiction, and that's really only because events described in the book haven't happened. Overall the scientific accuracy of the book is very good and launching subsonic robots with nasty payloads is far from one of the few iffy things in it. – Joe Kissling Apr 12 '17 at 13:02

In the current world, no standard-issue military or police rifle uses subsonic ammunition. There are lots of reasons for this, but in general this is because faster bullets fly straighter, do more damage, and penetrate armor better.

The principal advantage of subsonic ammo (and why some specialty cartridges like 300 AAC exist) is reduced noise. A silencer (suppressor, firearm muffler, moderator) can greatly reduce the initial report of a firearm, but the flying bullet itself makes a lot of noise if it's supersonic. Subsonic ammunition eliminates this sound.

If damage and armor penetration are not important in your future world (or taken care of by poisonous or explosive bullets as others have mentioned), perhaps subsonic ammo is favored for sound reduction. With a good silencer and subsonic ammunition, a shooter may be harder to find with surveillance equipment.

• The actual intent of 300 Blackout was to deliver AK-style punch without much retooling of an AR/M4 platform. It comes in both super and sub-sonic varieties, both throwing an absolutely massive projectile down range. What it loses in speed it gains back in weight. – UIDAlexD Apr 11 '17 at 17:40
• @UIDAlexD I assumed that reduced noise signature was at least one of the reasons for it, based on one of the early names "300 Whisper" – Deolater Apr 11 '17 at 17:44
• That's more of an incidental benefit. The projectile takes up almost half the cartridges length, and because it has to fit in a standard magazine it can't be made any longer. The larger projectile gives it decent ballistic qualities at long range despite the reduced speed. Since it was already a slow round AAC decided to market it as being good sub-sonic ammo, too. – UIDAlexD Apr 11 '17 at 18:25
• The 300 AAC was dual-purpose: supersonic bullets have the punch of AK-style bullets (while being more aerodynamic so they keep this punch farther out), subsonic bullets allow suppression and allow military and police to replace 9mm submachine guns for that purpose. And you can switch roles with clip change: no weapon surgery necessary. – Wayne Apr 12 '17 at 20:06
• 9mm submachine guns have been widely used by SWAT/SpecOps teams for close-in suppressed fire, and many people would view these over-sized pistols as "rifles". Technically you're correct, but... – Wayne Apr 12 '17 at 20:10

Let's bend the term "bullet" a little.

Instead of being designed to penetrate armour through force, the bullet could be a payload of plastique explosive with a shaped projectile that is fired at subsonic speed, hits the target, then detonates. Perhaps a thermite/phosphor mix charge that burns through armour and flesh.

• Nice. Since the damage is done by the shaped charge, you don't want it hitting the target too hard. – Mike Vonn Apr 12 '17 at 16:04
• Yep. Making it great for stealth ops. – Lu22 Apr 12 '17 at 17:52

To deal with the armor problem, consider the broadsword vs the rapier. The broadsword cuts, so armor protects. The rapier stabs, so the skilled swordsperson can find chinks in the armor.

So in this high-tech future, you have intelligent ammunition - basically micro-scale cruise missiles - which can steer itself to chinks in the target's armor. Subsonic both for quietness and to allow it a bit more reaction time.

# The Gyrojet (Handheld Rockets)

I'm surprised this wasn't mentioned yet, but have the gun could fire bullets with rockets behind thim. This isn't even new, it has been around since for at over 50 years. It is called the Gyrojet, and wasn't simply a prototype, but a full production line of firearms including rifles, pistols, underwater firearms, machine guns, and derringers. There were over 1000 of the pistol alone made.

While you don't expand on why you want them, I'll list several advantages.

## Quiet

The speed of the gyrojet was just around the speed of sound. Those that went below were reported to have made only a 'hissing' noise. With improvements to the technology and modern/scifi level manufacturing, it could be possible to make it even quieter or have no noise with improvements to the rocket nozzle. Or, you could possibly have the sound directed directly back to the shooter so no one else can hear it (Some stealth military jets direct noise/heat/light in as focused of a beam backwards as they can to avoid detection.)

With more development, the firearm might not even need a chamber (as traditionally used), so it could be possible to have it shoot more like a toy gun where it's discharged directly from the top of the magazine into the barrel. If this happens the striker, firing pin, or other ignition system could be operated completely by the trigger. This would make the firearm even quieter, as one of the loudest parts of a suppressed firearm is the action cycling. This could be improved on even more by having electronic ignition of the rocket. If this happens there are no moving parts beyond the rounds themselves.

## Reliability

Assuming you had electronic ignition, there are no moving parts to jam. The rounds are 'ejected' when they exit the barrel, so the only moving part is the magazine. Obviously this bodes well for reliability. Of course electronic could be augmented or made more reliable in many ways. They could even have the ingredients for a battery inside them and when they move up in the firearm they are 'primed' somehow by rotating or pressing on the back half of the round to mix them and you have a brand new battery. Or just have a capacitor that doesn't discharge over time integrated into the rocket. Caps get pretty small, could get smaller in the future. Maybe even have regenerative firing where the heat or gas pressures of one round charges up the weapon.

Or even have 'wireless' ignition. Have them stored in an RF blocking case/magazine, and they only reach a window where RF can reach them when it's 'primed' or ready to fire. Heck, you could even block RF from all but one direction. Send a pulse of RF that's enough to trigger a spark gap in the round and fire it. This allows it to be fired completely remotely with no gun at all (you only need the round).

Even if they are mechanical, only having a trigger activated firing pin is substantially simpler than all modern firearms, even revolvers, bolt actions, and single shots. Not having to deal with chambering and ejecting a round is fantastic.

Maybe even have both. Electronic ignition, with a primer as a backup. Have the trigger set off the electronic before the mechanical striker releases. No settings to touch, same action of pulling the trigger fires whether there's battery or not.

Cycling issues are also a very common cause of failure in semiautomatic firearms. Many of these will not exist if you don't have any moving parts on the firearm. I'm not sure it would be helpful to explain how the various failures happen depending on the action of the firearm, since we can assume this kind won't even have an action.

## No Recoil

Since there is no explosion pushing the round against the firearm, there is (practically) no recoil. The rocket is self-propelled. This can allow faster follow up shots with better accuracy. Additionally, this is more practical for low/no gravity situations. A regular bullet will push backwards with equal and opposite force. A rocket will only push backwards minimally while it is close by, and is self-propelled past that point.

## Flatter Trajectory

Regular bullets have an arc, because they are effectively 'thrown' from the gun at high speeds. The farther they shoot the higher the arc has to be. This also makes aiming long distances much more difficult, as you need to calculate bullet drop.

This could even have applications in some odd scenarios. Underground tunnel system where there are man-sized tunnels miles long? Might not be possible to arc a bullet without hitting the ceiling, but that wouldn't be a problem for a rocket.

## Intelligent Rounds

Being in the future it's not at all unreasonable to think there could be some logic in the bullets. This already exists, albiet in the prototype stage. DARPA's EXACTO round is one of two competing prototypes. It is capable of being shot out of any existing rifle chambered in .50 BMG. Considering that this already exists it's not unreasonable to think that the technology could be applied to handheld rockets too. It could redirect to 'miss' a friendly, or help hit a specific part that's being aimed for. Nonlethal could aim for an adversaries weapon to disable it. Being the future, most other non-ai weapons personal weapons like lasers likely won't be able to add much intelligence. Maybe even have it shoot around corners. The possibilities are endless.

One thing I was thinking would be interesting would be to use them like a tiny little mirv. They can use the rockets at the end to overcome wind resistance and go in at full speed. Just have a sattelite drop a few thousand from space and they can do whatever they need to on a large scale. Take out all power, phone, or fiber lines, even if they are buried? Shouldn't be a problem.

The underlying technology could possibly even be used for non-weaponized purposes. In my head I'm picturing a nailgun with intelligent rounds. Fire from across the job site and it nails itself up. Have some self-propelled bullets (They are just rockets, remember?) hold up a board while others nail it in. They only need to hold it for a couple of seconds, it doesn't seem that unreasonable. Okay enough with the tangent...

## More Power

Regular Bullets start off at full speed (after leaving the barrel) and decelerate the entire time. This also limits the effective range of subsonic ammunition.

Rockets can continue to accelerate throughout the flight, or even maintain speed. This allows a distance of travel only limited by the amount of fuel it can carry. Combine this with intelligent rounds and you could fire them from anywhere in fuel range, even hundreds of miles away (assuming you had a high tech dense fuel). They can also continue to accelerate after hitting the target, allowing them to hit multiple targets or possibly go through several layers of protection, as they can re-accelerate every time something slows them down.

This also allows more energy transfer into the target. The rounds can be as heavy or dense as needed, and the rocket can maintain just below subsonic speed. This, again, allows a stopping power only limited by the weight of the bullet. This may not be an issue at all for vehicle mounted weapons. An extremely dense very small round weighing a couple of pounds would be trivial to carry hundreds of on even a motorcycle, and a larger or military vehicle could carry thousands. This would be an extraordinarily deadly weapon. Even against armor you can combine this with 'Intelligent Rounds' above and it could find the weak points of armor on a tank or building and attack those. Since it's not really feasable to put extreme armor on every single portion of something, you won't need nearly as strong of an armor piercing round. Even tanks and "IED resistant" platforms use different armor on the front/sides vs the bottom. An explosion is different than an armor piercing round (A chunk of dense metal being thrown very very fast).

## Simpler, Lighter, and Smaller Weapon

Since there are no extreme pressures on the firearm, you can make the weapon out of nearly anything that could withstand the brief temperature of a rocket passing through the barrel. If we assumed electronic ignition, you could have a magazine and a barrel made out of high-temperature tape or plastic. Not that I'd want to rely on it being accurate, but the possibility is a semi-automatic version could possibly be made in seconds out of household materials, with no tools except tape needed.

Not needing any high-strength materials also lowers the weight of the weapon. This is always good.

Smaller also unlocks some interesting possibilities. You could have a single shot version with no weapon at all, only the bullet. If you can fit the bullet somewhere, you can fire it accurately from there. You only need a magazine to house other bullets, reload, things like that. You don't necessarily even need a barrel. Imagine having only a magazine, and a button on front that fired it. You could easily hold dozens more rounds on you than you can now. This also helps even more with the reliability aspect. If the 'magazine' is a self contained gun that fired rockets, you could have 3, 4, or 5 backups on you at any time. If there is a problem just drop it and grab another, like you were reloading. This is sometimes humorously called a New-York reload. You could have door knobs that can fire out electronically, or set traps, or have thousands of guns that fire simultaneously (even in 3d! Just have rounds behind each other that fire at the same time). Since each bullet is basically a gun by itself they can be fired without any supporting infrastructure (or even wirelessly). This effectively even allows you to use them to basically shape themselves into a larger object. Imagine them as a bunch of tiny little 3d pixels. I believe the mythbusters did something similar to paint a picture with paintball guns.

## Last thing I'll say

Most bits of technology have been available or working for years, and much of it was developed decades ago. It's not a stretch at all to think that some extra development or near-future tech could make it very useful for a long time or it could even become the standard firearm of the future.

Just put small rocket boosters on your bullet to give it consistent speed. This can also be used to make mini missiles which would give it the futuristic aspect. So they are slow, but heat seeking. You could do some interesting stuff with that. Maybe even have bullets that seek other bullets.

• Neat idea on bullet seeking bullets – Mad Physicist Apr 12 '17 at 17:25

In Harry Harrison's The Stainless Steel Rat series, the protagonist uses a .75 caliber rec'less pistol. It is non-lethal by choice. The weapon shoots different ammo types as the situation warrants. One could certainly find a use for tranquilizer rounds, "magnetic bug" rounds for tracking vehicles, exploding rounds if you aren't feeling non-lethal, something like a taser round, beanbags, a sedative gas round, etc.

Getting hit by a blob of putty moving at a high subsonic speed would take the fight out of most people.

Since the OP doesn't say it has to be a pistol, a short-barreled rec'less 20mm "cannon" would do the trick nicely.

Would they be useful? For bounty hunters, paramilitary, grey ops, detectives, etc, they could be useful as these people get in trouble for, you know, murder.

The Stainless Steel Rat was a detective/spy if memory serves.

• Technically he was a thief caught by the police and persuaded to work for them. – Tim B Apr 14 '17 at 8:42

## Make the bullet heavier. Done.

Impact energy is decided by the powder load, period. Powder = punch.

Imagine if the end of the bullet stuck out of the barrel and was held directly against the armor. The same powder would impart the same amount of energy to the armor regardless of the size, shape or weight of the bullet. What changes if the bullet travels some distance? Not the impact energy; the same energy bears against the armor (minus aerodynamic losses).

Th's a lot of math which can be done here, and you can do it if you really want to, but it's going to cancel out and lead you back to impact energy heing proportional to powder load.

For a fixed powder load, the mass of the bullet decides how much velocity the bullet will get. Make the bullet heavier, it'll get less velocity. Make the bullet heavier enough, and the speed is sub-sonic. So there you are.

## An armor-piercing slug is totally a thing

It is completely legitimate to have an anti-armor weapon which is simply a slug of metal. It is one of the major types of anti-tank rounds in common use today.

Then of course there is the classic mass driver from science fiction.

However, the trend is to make these rounds rather insanely fast. For instance the sabot round is almost a crossbow bolt much smaller than the tank barrel, the difference packed with discarding sabots, which fall off immediately after it leaves the barrel. The sabots are lightweight, so almost all the powder energy is transferred to the bolt.

The reason this is better requires a deep detour into the arcana of penetrating armor, which is too much to get into here.

If you're wondering if subsonic penetrators can be effective, half of all fatalities of a train hitting a car, pedestrian etc. happen with the train moving 4 mph or less. One engine with one car is about 200 tons, so that's 290 kJ, compare to 53 kJ of kinetic energy for an M61 Vulcan round.

## Slower and heavier is better... Kinda.

The bullet will lose energy from aerodynamic drag. The faster the bullet, the more loss. A supersonic bullet must spend energy making the sonic boom, so it loses more energy. A slower bullet brings more of its initial energy to the target.

A larger bullet will also tend to be more aerodynamic. One factor in aerodynamics is front-facing area. Another is wetted area, or total surface area exposed to the wind. These are area factors, but the bullet's mass is a function of volume. That makes the square-cube law work in favor of the larger bullet.

These factors mean the subsonic bullet will be slightly more efficient at delivering powder energy to the armor. That means better penetration, or less powder needed. I would say the farther the shot, the more advantage to the slower bullet.

## But not entirely

However range is a bit of a problem, as a slower bullet will have more of an arc in its travel, be more affected by the wind, and won't have the "legs" to go very long distances. This makes aiming more difficult, and may even force you into using artillery-style spotting and targeting computations. And the slow travel means spotter-based correction will take longer, and give your enemy more time to scoot.

It may well be that you take a shot and your spotter tells you "2 metres high, 5 metres too far right". And you adjust and shoot again. And for your enemy, it means if they're ever minding their own business and they hear an odd sound, to hit the deck / move their machine, because the next one will hit!

• "Impact force is decided by the powder load, period. Powder = power." The physicist is me is screaming. Forces are not energy are not power. Powder is stored chemical energy. This will be converted to kinetic energy (and heat and sound). But the impact force is a result of the bullets impulse. A heavier bullet gets more impulse out of the same kinetic energy. – MSalters Apr 13 '17 at 8:14
• @MSalters True. -- Better? – Harper - Reinstate Monica Apr 13 '17 at 14:40
• Much better, even. – MSalters Apr 13 '17 at 20:13

Short answer: yes, slow speed bullets are feasible.

You need the right tool for the job: you don't want a bullet that can go through five dudes and the wall behind when the only thing you want is to silently kill the one person in front of you.

A bullet at point-blank or close range doesn't need that much speed to cause the same damage that a high-velocity bullet can at long range. You could have private security firms or agencies use it when the job takes place in closed environments or when you don't expect high level treats.

Another reason, in line with the previous point, to keep using slow-speed bullets is cost. Even if high-velocity [HV] rounds have the same build, the reduced amount or different types of materials in production can still make these types of rounds more attractive for private companies or criminal groups who doesn't have the budget for more "advanced" supplies. You could dispatch with a couple HV magazines just in case. If you go with caseless ammunition, the cost difference becomes more significant (this goes for the weapons as well, as the gun doesn't need to be as sturdy if it is going to shoot bullets with less force behind them).

A bullet is a bullet no mater what. The last thing you want is to get hit by one. Armor is designed to reduce the threat, but the threat still exists; under a hail of bullets, it is only a matter of time until one find it's way through armor.

If what you want is armor piercing, you can use a high density material round. Look at the M829, is designed for armor piercing for tanks, but the design can be applied to smaller calibers.

Another point everyone is making is not to depend on trauma damage and instead use a payload: a chemical mix that activates upon impact, aka a HEAT shell essentially. Although these are described as explosive, in a sci-fi future, you can have a smaller, silent, "melting" charge. You only need a small amount of heat to cause serious damage to flesh, and, since you can shoot several rounds to the same target, that small burn could be enough to stop a threat.

• Welcome to WorldBuilding. I made substantial edits to your answer for grammar and spelling. It was hard to read (i.e. you used "treat" instead of "threat" in three places, which was kind of funny). You might pay attention to such details in the future. – SRM Apr 12 '17 at 3:34
• One thing concerns me about this answer... you don't specifically say that these bullets are slower-than-sound. You talk about slow-speed bullets, but I'm not familiar enough with ammunition... is HV always "above sound speed"? If you could take a moment to clarify, I think it would help your answer. – SRM Apr 12 '17 at 3:35
• "You don't want a bullet that can go through five dudes and the wall behind". That is exactly why you use a fast bullet. There are two chief ways to deliver the energy of the gunpowder to the victim: a slow&heavy bullet, or a fast&light bullet. (There's only a limited amount of energy in a handgun; a fast&heavy bullet would be a cannonball.). Of the two, the heavy bullet will plow through the victims. The fast bullet will dump its energy in a shockwave when it hits the first victim. – MSalters Apr 13 '17 at 14:25
• @SRM thank you for the edits, not a native speaker, sometimes words slip under the spell check. I'm not that knowledgeable either, I used those terms just to differentiate one from the other, is just a matter of reference, both bullets could still be under or above the speed of sound, but in this case HV(high velocity) is for above sound speed bullets while "slow-peed" are for bullets under the speed of sound, mind you, they still go pretty fast, so they are really not that slow. Also, is really annoying that hitting enter posts the comment, I had to edit and then re-post this 3 times. – Inferry Apr 14 '17 at 4:23
• @MSalters could you please elaborate on what your argument is? in the fifth paragraph I explicitly stated that you'll need a denser bullet if you want penetration. – Inferry Apr 14 '17 at 4:46

This is science-fiction, so we have to decide if the gun uses actual gunpowder. A Mass Effect-style mass accelerator firing a subsonic bullet could be made entirely silent. Supersonic projectiles would still produce a noticeable crack, even without muzzle blast.

Now, back to gunpowder. There is another type of subsonic bullet that every army uses and no-one mentioned: the 40mm grenade.

THUMP ----- BOOM

If your world is set in the near future, grenades could have shrunk a bit, say to 25mm caliber. You know, like the XM25 grenades...

https://en.wikipedia.org/wiki/XM25_CDTE

They make plenty of different ammo for this gun, but the sexiest is the HEAB round. It goes like this:

There is a bad guy hiding behind a corner. Aim optical rangefinder at corner, click a few buttons, aim slightly off the corner, and shoot. The grenade will detonate in the air, precisely just right in front of the bad guy's face. At least, in theory.

So, it defeats cover. Feel free to imagine various types of ammo, like heat-seeking, micro-missile, smoke, tear gas... There is even a 40mm round which you shoot up in the air, and then it pops a little parachute and the camera at the front streams live video to your HUD. Yes, you can find youtube videos of people playing with those right now.

A supersonic bullet produces a lot of noise when it breaks the sonic barrier. So for a gun with a sliencer it's good to use subsonic bullets. In the real word it should use a heavy bullet to effectively stop a target.

For example see this real gun.

Take a look at Arma 3's ASP-1 kir. it uses a 12.7*54 mm round. The gun itself is pretty useless beyond 300 m.

The main idea behind this gun is close range stealth (hence the build in suppressor) with lots of stopping power. The subsonic round means that no sonic boom is heard.

Subsonic penetrators might not be as effective as supersonic. but what about subsonic explosive? Or subsonic EMP? Subsonic biological rounds (or other rounds that might need to be subsonic to maintain effectiveness)?

edit: you don't really say why they need to be subsonic and what level of futuristic are we talking? Artillery shells can currently disperse biological agents but there is no reason why you cant get small calibre ones to be more selective - these could be subsonic to stop disruption of the agents.

subsonic HE rounds would pack a punch - whilst the firing might be stealthy, the impact will not be!

Depending on your level of futuristic, EMP technology might be built into the rounds. These could be stealthy ways of disrupting technology (i.e. not used against soft targets) in a sci fi way.

• Welcome to WorldBuilding! This looks like a good start for an answer, but right now it's a bit short and you don't really explain what you mean. Please try to flesh out your answer a bit more. For example: what do you mean with "biological rounds"? How would you make EMP-rounds? Do you know of any subsonic explosive rounds? If you have a moment please take the tour and visit the help center to learn more about the site. You can edit with the little "edit"-button at the end of your answer. Have fun! – Secespitus Apr 12 '17 at 10:27
• The question doesn't really say anything other than "futuristic bullets". There are no EMP bullets in existence at the moment but I imagine that if one was to be created then the calibre would be large and the contents fragile. The launching mechanism might even need to be howitzer style. The question asked for believability, my response would be change the intended effects of the ammunition to make it believable. Afterall the OP didn't say why they should be subsonic. The current-day primary use of subsonic ammo is stealth. – AngryCarrotTop Apr 12 '17 at 10:31
• Those are important points you mention in that comment that you didn't mention in your answer. Please add them to your answer to provide more content. – Secespitus Apr 12 '17 at 10:38

Sub-sonic bullets are meant for quiet, especially with suppressors/silencers, and so on. Lots of good info already given about the amount of kinetic energy and why you might want to go with supersonic rounds, so I'm going to skip over all that. Why? Because you can use your tech to make armor not matter all that much.

Even though various kinds of body armor can defeat all kinds of rounds, nobody is going to want to wear armor all-the-time. It's kind of uncomfortable. Even if your future technology makes it possible to wear armor most of the time, there are going to be things about it you can exploit.

Armor is always going to have holes or weak points. The neck, the face, and joints are all hard to effectively protect and still allow freedom of movement.

In a gritty sci-fi scenario you can simply use various kinds of augmentation, either direct implants or glasses, to link with a specialized handgun that calculates windage, range, bullet drop and such to make each shot land where you want. That means you don't have to go through armor, just around it.

So your film noir organized crime hitman can go after his targets with his new 15mm silenced pistol with Bullz-I (tm) augmentation integration to take out his targets.

This isn't going to be a normal military issue type firearm though. Maybe as a small side arm or as a special forces kind of thing.

I'd like to add one use for subsonic ammunition. It is much better-suited to sounds-suppressed weapons. The lower velocity imposes less wear on the suppressor and the resulting shot is much quieter (sound suppressors don't hide a sonic boom).

• Welcome to the site Mr Meems. While an interesting point this would seem to be more of a comment than an answer to the question that was asked. Please take an opportunity to edit and address the questions asked (keeping this relevant bit of information as well). To get an idea of how things work around here please check out the help center. – James Apr 12 '17 at 20:17

You could assume that your bullets are subsonic but can be launched in very quick succession with electronic triggering, like Metal Storm's million round/min gun, but perhaps scaled down a bit to have the same impact on body armour as one conventional high speed round.

A good example of a subsonic "bullet" is the blaster bolt in Star Wars. The "bullet" from a normal small-arms weapon (turbo-lasers seem to go much faster than sound) are noticeably slower than modern bullets, but seem to have a similar kinetic impact to a small grenade, being able to punch through military armor and kill even from close range impacts.

Taking this concept, your sci-fi weapons could fire some sort of charged particle/plasma similar to Star Wars blasters. In the movies, they do make an iconic sound when fired, but real-life energy weapons are mostly silent. Yours could be similar.

Subsonic rounds make a certain amount of sense in your scenario. Consider the following contemporary scenarios:

M1911 firing .45 ACP

The longtime standard sidearm is still in use by some special operation units because the of its subsonic round, which pairs nicely with a suppressor. Unlike reduced 9mm loads you don't have to give up any stopping power. Very appealing if you want a hard-hitting sidearm that works equally well quiet or loud.

M82A1 firing Mk 211

Definitely not subsonic, but an interesting case study in bullet mass. People often tout the .338 Lapua Magnum as a superior alternative to the standard .50 caliber sniper rounds, but the tungsten cored ones offer quite a bit more penetration. You could make a convincing argument that rounds such as the .338 can't be scaled up to penetrate future body armors without unacceptable recoil. Because tungsten is denser than lead switching to the larger rounds would come with lower velocities. That's still not subsonic, but if you scale down towards assault rifles consider ...

M1928A1, aka Thompson Submachine Gun

Say hello to my little friend! Did you know the tommy gun fires .45 ACP? Which we've already established is a subsonic round. There's a prototype for your futuristic, close quarters, .50 cal anti-material rifle right there.

Misc problems

First, nobody's buying a subsonic sniper rifle unless they're an assassin. They're unsuitable for long-range work, and less suitable than an assault rifle for close ranges. The only people who want them are using a suppressor at medium ranges.

Second, it's tough to imagine a really successful subsonic, compact submachine gun. Subsonic rounds around 9mm just don't buy you anything over the supersonic variety. There's a reason the Thompson went out of style, it's not exactly as easy to tote around as an MP-7, Uzi, P90, etc.

Finally, if it's not man-packable it's probably supersonic. Nobody's worried about recoil on a vehicle mounted 20mm cannon. You're going to make the bullet out of something dense like depleted uranium and pack it with a charge large enough to crack the moon.

If your world is set in an orbital habitat, space ships, or domed environments then it is entirely possible that all weapons would have a reduced penetrative power and muzzle velocity. This is to prevent the bullet going through critical electronics or exterior hulls/domes and threatening the entire environment!

If you want a slow bullet you have 3 options

1. Make the bullet very heavy. It will deliver a lot of momentum and energy to your target, and create pressure wave inside their body that cause internal damage.
2. Make the bullet do damage in some way other than impact. It could explode, poison the target, electrocute them, insult them...
3. Make the bullet very sharp. By distributing its impact over a small (molecule-small) area you can create an immense amount of pressure. If the bullet is made of harder stuff than the armor, it will punch a clean hole through and damage important things.

For the first option, you need something heavy. Depleted uranium would be a good option. Make the bullet as large as possible, preferably shaped like a modern bullet for aerodynamic reasons.

For option two, model it off of modern grenade launchers. You're trying to deliver something potentially fragile, so launching it in a canister is your best bet.

For option three, you'll want something heavy like in option 1, so depleted uranium is a good choice. The bullet still needs to be as heavy as possible because more mass means more pressure. You probably want bullets at least a foot long, tapering to a sharp point. The point needs to be machined to molecular perfection, but we're in the future so that's fine.

You can, of course, substitute some very heavy unobtainium for depleted uranium if you like.

In short, yes, particularly if "high tech" specifically means advancements on the concept of a bullet itself. A number of other answers have described mechanisms that could be slower such as delivery systems for explosives or poison injections. But lets generalize the concept and then focus on implementations that should be subsonic, effectively without tradeoffs:

Give the combatants (possibly guided) bullets with a second stage triggered on contact.

Explosives and poison darts fit this description, but with tradeoffs. Poison delivery is much more easily countered or shielded against than kinetic energy, and explosives create noise; likely a considerable amount.

But there are other options, such as an impact hammer (pneumatically fired slug, posibly supersonic but only traveling through solid matter), directed microwave burst, radiation burst, or nanobots. These need not increase the overall noise level and are likely to work better with lower energy impacts, so the secondary stage isn't rendered inoperable before it is deployed.

The impact hammer option is noteworthy as having the most similar effect to present-day single-stage supersonic slugs (in that it exclusively delivers kinetic energy and optimizes for minimum mass delivered).

My personal favorite is a laser cutter. In straighforward configuration, it simply cuts through the armor and flesh past the point where the bullet itself is stopped. More creatively, a fan of angled beam emitters around the back of the bullet might even be able to carve out a funnel of flesh between the time the tip impacts and the bullet stops moving (esp. rotating) entirely. It also presents what is likely one of the more realistic options in terms of technology/stored energy that must fit into a bullet-sized package.