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I have been thinking about realistic sci-fi small arms of future. Usually authors make use of lasers and other vague laser-like rifles. But if you think about it kinetic projectiles have a lot of advantages over lasers. In the future people might have figured out how to store and apply energy efficiently enough to accelerate smaller projectiles at much higher speeds.

So, in my world, handheld guns fire tiny needle-like objects (say weighing around a tenth of a regular bullet) at Mach 5, 10, 20 or 30. The two main benefits of this are painfully obvious:

  • Kinetic energy grows exponentially with speed and only linearly with weight, so it makes sense to decrease weight while increasing speed.
  • A needle is much smaller than a bulky bullet, thus allowing for much deeper magazines and requiring less load to carry on your body and hold in your hand. We are talking about orders of magnitude difference here.

Another big advantage with this system is that it allows the user to decide how much energy to apply. If the target is not big or armored, you only apply little energy and accelerate the needle to lower speeds, and vice versa.

For this discussion let us assume that the needles are made of a material that doesn't immediately shatter at such high impact speeds and that the entire construction has no additional issues like energy, heat or recoil management.

My question is regarding impact and effectiveness. Say I fired one of those bullets at a human or a bear. Would they be immediately stopped and neutralized? On one hand I imagine a hypersonic bullet penetrating a biological body at such a high speed would cause shockwaves inside, and turn inner organs into a soup, but it is also possible that it will simply leave a clean tiny hole which might kill the target but not immediately, making this a pretty ineffective weapon.

So, the question is which is it? What does an impact of such a tiny fast object look like on usual targets and would this be an effective weapon?

UPDATE: I also want to add that the exact design of this needle is not set. If you think that it should be made larger or smaller to become effective, please don't hesitate to play around with the parameters. I am just brainstorming.

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    $\begingroup$ The aerodynamics of needles flying at hypersonic speeds in the atmosphere would be very interesting. Especially at Mach 10 or above, it would be awesome to see. Could you please elaborate? $\endgroup$
    – AlexP
    Apr 6 at 10:21
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    $\begingroup$ Look up existing flechette weapons. There are some niche applications like underwater use, but of more interest might be that many modern mounted gun systems are moving towards fast needles/darts like APFSDS darts (Armor Piercing fin-stabilized discarding sabot) which are explicitly designed to defeat modern armored systems. $\endgroup$
    – Dragongeek
    Apr 6 at 10:53
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    $\begingroup$ If it doesn't shatter/deform at such high speeds, you're altering the physics of the bullet itself. At those upper machs, things behave more like a "liquid". Impact analysis relies heavily on how hypersonic rounds deform against armor and pierce them. Realistically any living thing shot with a weapon like that will be put down. But so will things behind it if you're going at mach 30 with a practically indestructible bullet. Something to look into: US Army's Special Purpose Independent Weapon system program. They had a similar idea to yours and it failed for various reasons. $\endgroup$
    – FIRES_ICE
    Apr 6 at 10:54
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    $\begingroup$ You might want to look at micrometeors and their effects on space stations. As projectiles get smaller, the issue starts to be how to transfer the energy to the target. So projectiles that disintegrate (frangible) will transfer energy and also reduce collateral damage by NOT penetrating hard materials. They may vaporize on/inside targets, leave tiny (potentially even toxic) shrapnel, etc. en.wikipedia.org/wiki/Frangible_bullet $\endgroup$
    – DWKraus
    Apr 6 at 11:21
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    $\begingroup$ A good general guide to these projectiles en.wikipedia.org/wiki/Flechette $\endgroup$
    – DWKraus
    Apr 6 at 11:32

6 Answers 6

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Dubious effectiveness

Disclaimer: This is all speculation. For obvious practical reasons, no one has conducted statistically valid studies on shooting humans or even animals with mach 30 hypersonic needles. (The entire science of wound ballistics has only stumbled gradually to its current level of knowledge on the effects of existing weapons by accumulating case studies.)

Before looking at possible effectiveness it is necessary to look at the problems with such a weapon. The first is that it needs truly extraordinary material science in order to launch the projectile without blowing up the weapon. With a conventional barrel that the needle is launched down, the air in front of the needle simply cannot get out of the way of the needle quickly enough. This means that without extraordinary materials the barrel will explode from the pressure built up in front of the needle, the same way a conventional firearm will blow up from excessive pressure behind the projectile if too much or the wrong type of powder is used.

Assuming that the barrel is made of indestructabilium, the air in front of the projectile being turned into plasma will result in increased energy usage (although the concept seems to rely on effectively unlimited energy anyway), extraordinary heating of the barrel that will necessitate a bulky cooling system, and the expulsion of the resultant plasma ahead of the needle, which will result in a high thermal signature.

Finally, it should be noted that both projectile energy and air resistance scale exponentially with velocity. This is a twofold problem - it means that even more energy is being wasted heating up air on the way to the target and that the actual impact velocity of a projectile will vary enormously with range. Which leads us finally to the downrange effects of the projectiles.

If a needle-like projectile drills a hole only a millimetre or two through a person, the chance of it swiftly incapacitating that person is pretty low. Yet it is quite likely to drill such a neat hole, since a needle which can remain intact while hurtling through the air at mach 30 will necessarily remain intact while pushing through a human body at lower speeds. The weapon somehow requires a projectile that can meet the following contradictory requirements:

  1. Projectile will remain intact while being accelerated to hypervelocity over a distance of less than a metre (assuming a bullpup design, conventional layout makes the barrel length even shorter - and do not even think about pistols)
  2. Projectile will remain intact and travel in a "straight" ballistic arc at hypervelocity speeds
  3. Projectile will remain intact and penetrate body armour
  4. Projectile will shed all/most of its velocity over the space of 15 cm once in human flesh, whether or not it has encountered body armour.

Therein lines the problem - a projectile that can survive as required by points 1, 2 and 3 is going to punch a little hole in human flesh and keep going with most of its kinetic energy rather than disintegrating in it and creating wound effects. There are ways around this dilemma for slower, wider projectiles but I do not believe they could be engineered into a needle that can survive the extraordinary requirements of being accelerated to and travelling at hypersonic speeds.

It is worth noting that since switching from 7.62 mm to 5.56 mm projectiles that the US are now planning on switching to a 6.8 mm series of weapons due to insufficient combat effectiveness from 5.56 mm. (They considered a flechette-shooting version of the Steyr assault rifle a few decades ago in the advanced combat rifle trials and decided against it.) If you want your weapon system to be more credible then have your man portable gauss rifles shooting rounds with the same calibre and comparable velocities as those the US forces are looking at. They will have the advantage of being caseless, low signature weapons, meaning that even bullpups can be fired from either side of cover and ammunition capacity could be almost doubled (assuming the fictional power source has negligible weight).

The ability to "dial in" the desired power is probably not an advantage in most practical situations. Soldiers in the heat of battle will seldom know if their next shot is going to be attacking an unarmoured target (or part of a target), an armoured target or a target on the other side of soft cover. The one situation in which it would be useful would be for special forces conducting a stealthy operation where they could switch to subsonic shots until/unless an alarm is raised at which point they could go to full power. However, small needles would be so ineffective at guaranteeing sentry kills that this would again necessitate a larger calibre weapon.

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  • $\begingroup$ I'm really curious. Kinetic weapons are all about transferring a certain amount of energy from the shooting weapon to the target using a projectile. Modern small arms shoot with kinetic energies of around 500 Joule. If I were to make the bullet extremely small and shoot it at extremely high speeds (say with a railgun design) such that the final energy is still 500 Joule but with deeper magazines, why would this create overheating and plasma problems? After all there is only 500 Joule in the system which when fired by a modern gun is not enough to create plasma or explosions... $\endgroup$
    – Hrach
    Apr 6 at 17:15
  • $\begingroup$ I disagree about soldiers not being able to switch between different modes in the heat of the battle. Sometimes they might not be able to but there are a lot of scenarios where a target cannot be taken out by conventional small arms so MANPADs, ATGMs, RPGs and air strikes have to be requested. If technically feasible, the soldier who is on that target should be able to quickly switch the parameters of their weapon and attack again rather than radio someone else and wait. $\endgroup$
    – Hrach
    Apr 6 at 17:22
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    $\begingroup$ @Hrach: Because of the hypersonic speed. Air behaves basically like a brick wall when something tries to fly through it at hypersonic speeds. (And an unpowered lightweight object flying at hypersonic speeds will be brutally slowed down to subsonic speed in maybe one meter.) $\endgroup$
    – AlexP
    Apr 6 at 21:08
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    $\begingroup$ @Hrach it's worth looking at what happened with the 5.56 mm SS109 round. The original round was designed for firing through full-length rifle barrels. Then the US special forces decided that the M4 with a super-short barrel was a great idea, but without a long barrel for the acceleration to occur in, they needed to use "hotter" ammo to achieve the same muzzle energy, resulting in the M4s only having a service life of a few thousand rounds. That's to achieve a muzzle velocity of <Mach 3, where you want Mach 30 - higher velocity = exponentially higher stress on launcher $\endgroup$ Apr 7 at 12:17
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    $\begingroup$ @Hrach: Being small and lightweight is not an advantage when trying to fly at hypersonic speeds, but rather quite the opposite. What I really expect to happen is for the tiny 10 milligram piece of metal to ignite and burn in an instant. $\endgroup$
    – AlexP
    Apr 8 at 12:52
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Ballistic effectiveness is more a function of how much of the energy is dumped into the Target

Now - Caveats - I have never been shot and have no intention of being shot...

That said...

a Bullet that passes straight through with very little in the way of deformation is more likely to be surviveable as most of the kinetic energy isn't dumped into the target whereas a Bullet that deforms/expands and dumps all of that kinetic energy into the target - that is the one that is more likely to kill you.

a Needle, even at hyper sonic speeds is more likely to just pass straight through, whereas a bigger projectile that is travelling slower speed is more likely to be devastating to the target.

Therefore - I don't think they will be as effective as you might hope.

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  • $\begingroup$ A good analogue would be getting hit by a particle accelerator beam. The beam carries a lot of energy, but it has no time to actually distribute through the object it hits. So you end up with a tiny hole and not much else. $\endgroup$
    – ErikHall
    Apr 8 at 16:58
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You are basically describing flechettes

In the late 1980s, following the promising results of reducing the 7.62mm round down to the smaller, faster 5.56mm rounds they use today, the USA experimented with further reducing small arms slug sizes through the use of saboted ammunition.

A saboted round consists of a thin, hard, dense dart called a flechette encased in a disposable, lightweight casing called a sabot that allows one to concentrate a significant amount of gunpowder into accelerating a narrower, faster projectile. When the sabot leaves the gun barrel, it breaks away leaving just a high speed dart.

enter image description here enter image description here

While this technology is very effective at allowing large calibre cannons to achieve better armor penetration making it good as an anti-material weapon, it was found to be counterproductive when used as a small arms weapon. As such, no modern military uses flechettes for anything smaller than 12.7mm guns.

Why you should never use flechettes for small arms weapons

Over-Penetration is a terminal ballistics term referring to what happens when a projectile has more kinetic energy than it can impart into a target; so, instead of stopping in the target, in penetrates out the back and all of the remaining speed that it has is just wasted energy. When a 5.56mm round hits a person, it is balanced to have just enough kinetic energy to pass through your body, and not have any significant remaining energy to exit the other side with. This means that the bullet is designed to impart all of its energy into causing harm. If you were to replace a 5.56mm slug with 1.85mm flechette, then you'd have 9 times less cross section; so, your flechette would impart up to 9 times less energy into the target before flying out thier backside. No amount of additional velocity would increase the amount of matter your dart is interacting with. Also, any added benefit you get from an increases thermal shockwave would be offset by the added heat better sealing off the wound. Instead of a lethal blast, what you'd get is a small, clean, cauterized, wound that would be unlikely to incopacitate, much less kill the target.

The second reason not to use flechettes in small arms is collateral damage. Armor piercing rounds of any variety pose a greater risk of collateral damage. The main reasons most police forces are required to use hollow points is not just because they have more stopping power, but because overpenetrated rounds and stray fire is less likely to go through a target or wall harming a bystander on the opposite side. If your infantry is shooting off hypersonic rounds, they are more likely to cause civilian casualties. And modern information warfare makes maintaining the perception of a war being righteous far more important than having bullets with a bit of extra penetration power.

Why flechettes were never adopted for penetrating body armor

The only advantage of using high velocity flechettes over traditional slugs is thier ability to penetrate armor better. However, we will never see flechettes used this way because they are already obsolete compared to modern armor penetrating rounds. Instead of having a sabot that is ejected from the flechette when it leaves the barrel, modern anti-armor small arms bullets use a hardened steel or tungsten penetrator encased in a soft metal like lead or copper.

So, when an armor penetrating round like a 5.56mm green slug hits a soft target, it still has it's full 24.28mm of surface area to impart energy into its target, but if it hits something like a plate of body armor, the soft outer sabot breaks away and the harder, denser penetrator pierces the armor just like a flechette giving you the best of both worlds.

What a hypersonic small arms railgun slug should really look like

Since you want to break known material science anyway to make this weapon feel more future tech, let's just throw out all the concerns of what is survivable, stable, etc. and just focus on what would work best "with the right material".

Instead of going long and skinny, you want your slugs as short and fat as possible. An ideal shape would probably be a conical metal disk with with an outer ring just long enough to engage with the barrel or rail system, and at the very tip you can have a small high density penetrator for dealing with armor. Much like the green bullet design described above, this form is designed to spread out the impact on soft targets, and concentrate it on hard targets, but it uses WAY less material to be able to interact with a given cross-section.

While a disc like this would do little harm at present small-arm velocities, at your higher railgun speeds, they would be able to to fully penetrate a soft target. At this size, your gun could hold hundreds of high calibre rounds in the same space and weight of a normal sized ammunition clip.

enter image description here

The trick will be overcoming air-resistance at such high speeds, but heck, we're already talking a future tech weapon system with a massively powerful energy source and as of yet undiscovered materials; so, handwaving a solution to air resistance should be the least of your problems.

The best option is if the weapon can create a vacuum for the slug to fly in. There is already existing research into lasers that can create a cylindrical vacuum by firing a very strong but brief laser pulse. Combine these technologies, and your disk could be fired through such a momentary vacuum allowing it to not just achieve mach 5+ inside of an atmosphere, but maintain that velocity as it flies into its target.

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  • $\begingroup$ Thank you for mentioning the vacuum generating laser. I’d have needed to leave another answer if you hadn’t. $\endgroup$
    – DWKraus
    Apr 8 at 23:06
  • $\begingroup$ Eh, there is explosive ammonition, and with a well enough designed timer, the projectile could self-destruct on impact, post-penetration. $\endgroup$
    – Pica
    Apr 9 at 9:12
  • $\begingroup$ @Pica Because it is a hypersonic weapon, an explosive can not expand fast enough to be all that useful. An explosive shell can't just be a flat wafer meaning you need a longer slug which means you hit with enough kinetic-energy-to-surface-area to over penetrate. Even if you start the detonation on impact, a high speed slug will only get to expand a few millimeters before it all flies out the back of the target. Explosive small arms bullets are also a war crime. Also, the OP wants to increase ammo capacity; explosive bullets would be bigger than kinetic discs. $\endgroup$
    – Nosajimiki
    Apr 9 at 15:27
  • $\begingroup$ Not much, you just fire a 2nd thinner instance inside the hypersonic bolt, timed to hit the tip, just post impact. If you are that fast, you do not need that much explosives. All that momentum turns into heat, cavitation and fragments. $\endgroup$
    – Pica
    Apr 9 at 17:09
  • $\begingroup$ @Pica there is also the issue that a railgun this powerful will significantly heat, electrify, and compress your slug; so, there is not really a good way to put electronics on board for timing the shot, or any explosive that I know of that won't be detonated in the barrel. $\endgroup$
    – Nosajimiki
    Apr 9 at 19:33
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Spectacular on Paper, A liability to the User in Practice

These darts are traveling at just under earth's escape velocity, so you're basically talking about launching a very small meteor at someone. Depending on composition it should have more than enough energy to vaporize itself on impact and will likely burn up as it travels. As long as it's properly aligned with its direction of travel and the whole projectile, with whatever it has for fins, fits within a 3 degree wide cone, it should be able to cover a useful distance before melting and disintegrating.

On impact, my money would be on it rapidly vaporizing, and basically exploding as it traveled through the target, probably having an effect similar to a small grenade going off inside the target. If this projectile is sized to have recoil similar to a 'battle rifle' each round will be carrying about as much energy as a hand-grenade at the muzzle.

The downside is that if the projectile is disrupted in any way, it'll tumble, causing a massive increase in drag and heating. Once it starts to tumble, it'll be torn apart by the mechanical loads from drag, and the thermal loads will start to melt and vaporize it in mid air. These effects working together create a positive feedback loop. As the dart breaks up its surface area increases, causing an increase in drag and heating. As it heats and then melts, it gets softer, causing it to break up more rapidly. This process can take place so rapidly that it effectively results in an explosion. If you've ever heard of asteroids or comets exploding as they enter the earth's atmosphere, that's basically what happened. The asteroid started to break apart, and the sudden increase in surface area caused it to turn kinetic energy to thermal energy so quickly that it explosively vaporized. Given how much energy these projectiles may be carrying, the effect of one burning up would probably be similar to a concussion grenade going off.

This means that if the projectiles have defective fins or deform during acceleration or are even the slightest bit rotationally asymmetrical they will probably start to tumble, break up, and vaporize the moment they leave the barrel, probably killing or injuring the shooter. If the tip of the projectile doesn't burn off evenly, it may cause it to start to tumble and then explode at a seemingly random distance from the shooter. If the projectile grazes a leaf or bug, instant unpredictable airburst.

It also means that for this weapon, concealment is nearly as good as cover since the projectile is extremely unlikely to penetrate past the first thing it hits. A sheet of drywall or plywood would cause the projectile to break up and explode, so while you wouldn't want to stand directly next to a wall under fire from these weapons, as long as you stood far enough back they couldn't harm you directly (They could ruin your hearing, and if you're indoors the overpressure may pulverize your lungs, though). In a forest, hiding in dense foliage may offer decent protection. A beefed up Whipple shield (multiple layers of thin metal foil normally used to protect spacecraft from micro meteoroids) could potentially soak up fire from this sort of weapon all day long. It'd be destroyed in the end, but a regular rifle would just punch straight through with little difficulty.

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    $\begingroup$ As per the OP's scope, we are to assume the the needle has the material properties required to survive being fired at these speeds. As long as the flechette is durable enough, any mid-air tumble will be corrected by the fins. $\endgroup$
    – Nosajimiki
    Apr 8 at 17:02
  • $\begingroup$ @Nosajimiki: How exactly does fin stabilization work on tiny ultra-light hypersonic projectiles? $\endgroup$
    – AlexP
    Apr 10 at 0:25
  • $\begingroup$ @AlexP Fin stabilization is based on drag. The fins exert more drag than the tip causing it to "slow down" at a higher rate than the tip. This pulls the dart into alignment. The faster you move through air, the more drag you experience and the stronger the pull becomes that forces the dart into alignment. So, fin stabilization is more a function of how much air you've passed through than it is of how fast or far you've gone. $\endgroup$
    – Nosajimiki
    Apr 10 at 3:17
  • $\begingroup$ Fin stabilization on a hypersonic projectile works the same way as on any supersonic projectile. There's no difference in the fluid flow principles, the only reason we make a distinction is that conventional air-breathing engines can't operate at hypersonic speeds. The fins act like wings and generate lift if the projectile is not aligned with the air stream. This lifting force is oriented toward the direction the projectile has turned(dart pitches down, fins generate downward force). When this force is applied behind the CG it tends to pitch or yaw the dart back into line with the air flow. $\endgroup$ May 7 at 18:23
  • $\begingroup$ The dart also needs to generate more drag behind the CG than ahead of it, or the dart will tend to want to flip end over end. Fins can provide this drag, but so could a solid cone at the end of the dart. Fins can also impart a slow spin to the projectile. This is not bullet-like spin stabilization (at those rotational rates a projectile with a high aspect ratio like a dart will tend to be destabilized rather than stabilized). Instead a gradual spin prevents manufacturing defects from causing the projectile to veer off course, causing it to corkscrew instead. $\endgroup$ May 7 at 18:30
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Science fiction has historically referred to this as a needle gun.

Not to be confused with the things that make tattoos, a needle gun will fire a continual stream of high-speed bits of metal of varying size. In some implementations, I've seen the weapon described as scraping bits off of a solid chunk of metal and propelling them via a coil or rail gun.

As Nosajimiki mentioned, an individual projectile of this size will enter a body and pass through the other side without dumping much of its energy. This presumes a long, thin needle-like projectile, but that isn't exactly necessary.

If, instead, you are shooting tiny bb's at high speed, then you don't have all of that excess energy. You lose the armor-penetrating qualities, but the high rate of fire means that you're literally perforating the person.

The downside of this design is that the stream will suffer a lot from air resistance. The range would be limited and the accuracy would be poor, like a shotgun. If your world had electrical storage that was lighter weight than chemical energy storage, it might actually be worth it.

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FRAME CHALLENGE:

For the various reasons given in the responses to your question the ballistic & mechanical characteristics of your 'needle rounds would probably tend to make them 'sub optimal' in terms of material damage & wound severity. So it might be better to abandon the 'heaps more rounds in the mag' approach and accept a significant reduction in rounds in exchange for better stopping power and longer range. Otherwise unless your talking close range personal defense or hold out pistols etc your characters are going to be stuck in a lot of 'spray and pray' fire fights.

This being the case it might be better to SABOT your needles so that one 'round' fires a cluster of needles (5 to 10?) contained inside an aerodynamic carrier. This carrier delivers the needles to the target before breaking up or flattening on impact and releasing the needles. Depending on how many needles are clustered together in each round this approach would give you the range advantage and penetration of convention rifle bullets with the damage form factor of (shorter ranged) shotgun rounds.

Below is a link to a product called the Galser Safety Slug.

[Glaser][1] [1]: https://en.wikipedia.org/wiki/Glaser_Safety_Slug

As you will see Glaser rounds used bird shot as the 'filler' but in your case the sub-munitions could be something like tungsten flechettes. So your 'bullets' will need to be longer relative to their diameter and probably heavier than normal bullets of the same caliber. And if that extra mass means you're restricted to the lower velocities in the range of muzzle velocities your provided? Even 'just' Mach 5 should be devastating! (At sea level sound travels at a speed of 331 meters per second so your bullet's are going to have a muzzle velocity higher than that of the M1 Abrams 120mm main gun! Recoil compensators are going to be a 'must have' BTW.)

You also should note that while soldiers absolutely value magazine capacity in combat (up to a point) they tend to value stopping power, accuracy and lethality more if the trade off is capacity is manageable. The additional advantage from your perspective is that this approach would let you fire other types of rounds from the same firearm, anything from tracer, to low velocity/non-lethal 'plastic' bullets' & paint rounds. Anything you can think of that will fit inside the chamber? The world is then your oyster in terms of bullet selection.

P.S. You will also need to assume that no such thing as the Geneva Convention exist in your world. From memory 'explosive' bullets are a no-no under the convention and I have no idea whether these rounds would qualify. (As always happy to be proven wrong on this point.) But that said and depending on the number of needles included in each round the effect of being hit by one, at the velocities you describe would certainly be 'explosive'.

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