Future Firearms Ammunition Technology 004: Flechettes – Why Fire Bullets, When You Can Fire Darts?

Flechette rounds, left to right: XM645 with glass polyester sabot, XM645 with compressed sabot, XM110 with GP sabot, 10gr SPIW flechette and sabot above, XM144 with GP sabot, 10gr SPIW flechette above, .330 Amron Aerojet triple flechette, 9.53x76mmR Winchester quadruple flechette, AAI 5.56x45mm ACR flechette, Steyr 5.56x45mm ACR plastic cased telescoped flechette.

Flechette rounds, left to right: XM645 with glass polyester sabot, XM645 with compressed sabot, XM110 with GP sabot, 10gr SPIW flechette and sabot above, XM144 with GP sabot, 10gr SPIW flechette above, .330 Amron Aerojet triple flechette, 9.53x76mmR Winchester quadruple flechette, AAI 5.56x45mm ACR flechette, Steyr 5.56x45mm ACR plastic cased telescoped flechette.

In the last episode, we discussed how the most ballistically efficient projectiles are the longest, most slender ones, with the highest sectional density. This naturally leads to the idea of using a super long, rod-like projectile which would in theory have excellent ballistic characteristics… But there’s a problem with that: Unfortunately, modern rifle projectiles are spin-stabilized, and there’s a limit to how long of a projectile can be and still be stabilized by gyroscopic forces in that manner (that limit is about 7 calibers long in theory, more like 6 in practice). This means that to successfully stabilize the longest possible projectile with the highest possible sectional density, another method is needed. The most popular alternative – and the one used in arrows, darts, APFSDS tank projectiles, and today’s topic, flechettes – is fin-stabilization.

Fin-stabilization is a form of drag-stabilization, where the projectile is shaped in such a way that the drag keeps it properly oriented throughout its flight; in fin-stabilization this is accomplished by adding fins. The result is a small arrow-shaped projectile with fins at the rear (called a “flechette”), virtually always coupled with a sabot. In this configuration, a lightweight flechette can be fired at extremely high velocity on a small amount of propellant, while producing ballistics that put even the flattest-shooting wind-buckingest conventional rounds to shame. In addition, the projectiles – having very high sectional density and striking velocity – possess excellent penetration characteristics. Also, very long, lightweight projectiles (~10 gr/0.65 g) can be used, allowing a dramatic reduction in ammunition weight versus conventional designs.

0824160050a2

This photo illustrates how an SPIW-style flechette works in conjunction with its disintegrating sabot. The sabot and flechette are fired from the case as a unit, but once this leaves the muzzle, the sabot disintegrates, leaving only the flechette to travel to its target.

 

Flechettes may have excellent ballistics, but they also have some limitations. Small flechettes retain energy well, but due to their small diameter do not create very large shockwaves as they pass a target, and therefore have poor suppressive capabilities. Their low volume, too, is a hindrance to their lethality, although flechette projectiles have proven to distort and bend, “J hooking” readily in tissue. Also, for very high velocity flechettes (over 4,000 ft/s), the projectile itself must be able to withstand the extreme acceleration of firing, which was a problem for the SPIW program of the 1960s and 1970s. Further, the manufacture of very small flechettes that could withstand high acceleration forces never became very mature, and historically small flechette ammunition was relatively expensive to produce.

Still, it’s not difficult to imagine a weapon firing low-mass, high velocity flechettes coupled with a lightweight case design and an extremely high rate of fire. Such a device could possibly compensate for the low effectiveness of its individual projectiles simply by firing much more of them than a traditional weapon, and the extremely low ammunition weight would facilitate this prodigious expenditure.



Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


Advertisement

  • Amplified Heat

    I hope “squeeze bore” gets covered as a topic, since it carries many of the benefits of sabot type projectiles (high velocity, low mass, very high BC projectiles) without the drawbacks of wasted sabot weight or a need for fin stabilization if rifled properly. A few early attempts at small arms usage was done for the SALVO project if memory serves, but were pretty crude.

    • Rodford Smith

      IIRC, squeeze bores have accelerated wear. Modern materials science could offset that, of course.

      • Amplified Heat

        Compared to standard chamberings, or to highly necked magnum pressure barrel burners that would be the nearest ballistic analog?

        • El Duderino

          You makin’ fun of my custom .204x99mm varmit buster?

    • It’s scheduled for tomorrow:

      http://i.imgur.com/nkt9xiq.png

  • Isaac FluffyWolf Rader

    I think I remember a mention that these actually deflected off of rain.

    • ostiariusalpha

      That seems to be more of an old wives’ tale, though I’ve told it myself more than a few times. Really, any raindrop would be vaporized instantly by the projectile’s supersonic bow wave; even assuming that the tiny dart would actually strike a drop, which isn’t as likely as you would think. Now how the projectile’s trajectory would be affected by light cover, such as windshield glass or sheet metal, is another matter.

      • Kivaari

        You should live in Grays Harbor WA. It really rains there. Better yet try Forks where it hits 160 inches per year.

      • FarmerB

        I don’t know the definitive answer to this, but I’m sure the deflection characteristics of fin stabilized projectiles is going to be vastly different to spin stabilized. I’m guessing not in a good way, either.

  • The Steyr ACR combined two of the “lightening the load” practices by using a polymer, cased telescopic cartridge to fire saboted flechettes at 4,750-4,900 fps. Cartridge weigh was 50% below that of regular 5.56, and the design allowed for 3-round hyperburst at 2,200rpm.

    And that was in 1987.

    • Giolli Joker

      I prefer their AMR (IWS2000)… anti-materiel use can be more cost effective in employing expensive solutions as FSDS or flechette rounds.
      For general use flechettes will always be fascinating but almost impossible to be realized:
      -in small calibers;
      -in large volumes;
      -accurately;
      -at a cost comparable to conventional alternatives.

      Saboted projectiles as introduced in “Chapter 3” however could have more luck, imho.

      • The AMR/IWS2000 is another one of those incredible future weapons from Steyr. They were really crushing it from 1977-1990.

        As for flechettes, I’m sure it could be done, but I agree, I think saboted conventional bullets provide the best 80/20 solution.

        A polymer cased telescopic round, firing a saboted 77gr high BC .224 at 3,300 fps, seems like it would offer much higher performance than 5.56, without the excessive weight and recoil that comes from moving up to 6.5.

        Combined with smart optics like the Steiner ICS, good hits out to 800 yards would be attainable for regular riflemen (assuming the threat is polite enough to stand still.)

      • EgregiousCharles

        All of those objections are things that will be addressed as manufacturing advances.

        • Giolli Joker

          It’s the cost issue the one that would still be VERY hard to be solved.

          • EgregiousCharles

            Not really for two reasons: 1) it just has to become cheap enough, not comparable. Rimfire cartridges are still way cheaper to make than centerfire but rimfire isn’t used much because centerfire is better and cheap enough. 2) Precision manufacturing (as for accurate saboted steel flechettes) becomes cheaper and cheaper while rare metals (copper and lead) just become more expensive.

          • Giolli Joker

            I’m only a humble mechanical engineer not involved in ammunition manufacturing, therefore I may be wrong, however, my 2 cents:
            1) both mature technologies, unlike in this case; the cost-benefit ratio for centerfire over rimfire is highly in favour of the former, with flechettes against conventional projectile the advantage is still to be demonstrated.
            2) limits are not simply in technology, but in physics… you need a sabot light but stiff enough to properly keep a glorified needle straight and perfectly aligned under accelerations able to push the load from 0 to 1.4 km/s in half a meter; a weight tolerance of 2 grains over a 50grs bullet it’s 4% of its weight, over a 10grs flechette it’s 20%; alignment and concentricity become paramount; perfect loading in the casing would be critical. MAYBE machining individually each component we could get good accuracy, but cost and TIME (=cost) would be very high. If we were talking about sniper ammo in .50 BMG I could see the effort (BTW, check the cross section of the Steyr 15.5 AMR round), but here the discussion is on assault rifle rounds… Indeed, IIRC, SPIW flechettes were designed with an eye towards mass manufacturing, by stamping of the tail, this step was probably cost effective, but accuracy never came.
            Personally I love the concept, but I’m very dubious on its application in small arms but in very limited (and large caliber) niches.

    • Arathar

      Not just that but a rising chamber and a frigging long bullpup barrel. Compared to that the CSAS with its tiny barrel and hilarious design is outdated since 3 decades.

      • Yeah, I’d say that overall the ACR is as close to a laser gun as we’ve yet created.

      • mig1nc

        Ambi ejection too. Way ahead of its time.

  • Kivaari

    The only professional reference I have on these is from Dr. Fackler, circa 1989. He was opposed to them. He claimed the bending and distorted darts were the result of not hitting tissue but hitting other intermediary targets, the “blade of grass” or brush. He said the claims made by the other Army Arsenal doing the work had taken surgical reports from Vietnam and concluded the nasty wounds happened post tissue contact. Me, I don’t know. I just don’t like the idea of these little darts.

    • One of the primary reasons I included flechettes is that if ceramic body armor becomes common, there won’t be a whole lot of other options for small arms ammunition.

      • Kivaari

        I had not thought of the ceramic armor issue.

      • Amplified Heat

        Interesting angle, for sure. I favor the squeeze bore concept, since it is basically approximating the function of an Explosively Formed Penetrator that we all know works quite well against armor, which if miniaturized to small arms size, would be akin to a true directed energy weapon that’s still man portable.

        • Energy retention for squeezebores is a problem, though. Flechettes retain their energy extremely well.

      • Warren Ellis

        Er with ceramic body armor, much like what most modern militaries use, can’t you just shoot them several times to crack and ruin their ability to protect you?

        I mean when the ceramic is getting cracked and broken, I imagine its protective qualities reduce dramatically.

        • I hear hitting somebody once in combat is a big challenge, so hitting them five or six times isn’t something anyone should be counting on.

          Plus, expect armor to get tougher, and more resistant to multiple hits.

    • Arathar

      Easy to solve, take bigger darts.

      If we all would use flechettes, and someone wouls show us a 7.62×25, noone really would like it and rather be sceptical. While fast, high bc 6mm cartidges work well.

    • ostiariusalpha

      Those were artillery “shrapnel,” which is a different ballgame from smallarms’ projectiles. The buckling of the flechettes in tissue analogs like gelatin blocks was well researched and establish at the velocities that the SPIW program was looking at. That same research also did a better job than Fackler at showing that the velocities of the artillery flechettes was inadequate for creating serious wounds.

  • Blake

    I’d love to get a bit of clarification regarding the statement that they “do not create very large shockwaves as they pass a target, and therefore have poor suppressive capabilities”.

    I was under the impression that suppressive fire was just slinging a ton of lead towards the target in an effort to keep their heads down. Are you saying that they just don’t make much noise during flight (compared to the zips and cracks of traditional bullets) so potential targets wouldn’t even know they were flying by? Or is there a piece of the puzzle that I’m missing?

    • ostiariusalpha

      I’m not sure what Nate was referring to, but there is a weird fact about supersonic bodies: in that the greater their velocity beyond the transonic threshold, the tighter the shockwave cone becomes, and therefore lower perceived volume to a listener.

      • Also their shockwave is in some way proportional to their size and drag. SPIW-style flechettes are very small and very low drag, so we wouldn’t expect them to produce very “effective” shockwaves from a suppression perspective. Empirical testing bears this out; small flechettes have almost negligible suppression effect unless they are right on you.

        • Amplified Heat

          You could *maybe* increase the effect with either a blunt nose (though a bow vs oblique probably isn’t a huge difference for something this size vs. an observer’s distance), or more likely playing with the compression/expansion wave timing to get a kind of “boom” amplification effect, at least nearby before they coalesce. You can sort of cancel out the boom at a given velocity this way, so it stands to reason you can amplify it as well. Maybe one of Homer Simpson’s “loudeners” as a muzzle device, lol. An explosive payload, even small, would do wonders as well.

          • noob

            will the government pay *us* $200 every time we transfer a “loudener”?

          • Giolli Joker

            An explosive payload on a flechette 1.8mm thick? 🙂

        • Kivaari

          I would think that near misses, but hits on hard surfaces like rocks and steel could be dramatically less with these than “real bullets”. Just watching the cloud of lead and copper when a steel plate is hit with a 5.56mm compared to a 7.62mm is dramatic. The material thrown into the air has to have an effect on nearby personnel. I know being near impact zones when a bullet blasts into thousands of shreds is hazardous to anything close.

    • FarmerB

      No, that’s it. If you hear bullets cracking around you, you’re going to stay under cover.

  • Anomanom

    Because shooting people with skinny needles will not cause much damage.

    • ostiariusalpha

    • Tim Pearce

      …until you get the velocity high enough. That’s a whole different challenge, though.

  • guest

    “future”… more like the past.

    These kinds of things were not adopted for a reason.

    • Guest 2.0

      Things can be solved. With a lot of passion and time.

      Completly redefined and perfected they wont look like the past anymore.

  • wetcorps

    You’re too damn productive Nat I can’t keep up with your posts 🙁

    • Gotta post post post

      • Daniel M. Ramos

        Don’t pay attention to him. Give us more. 😃👍🏻

  • jerry young

    In Vietnam we had rockets filled with flechettes it was like firing a large shotgun round hundreds of flechettes would cover a large area there was even a dye in them their intended use was to mark an area where the enemy was supposed to be not to mention the destructive force delivered!

  • Nicholas Trueblood

    works great in shotshells. cause they arent spun in a shotgun the fins stabilize them better. Much like the m1’s APFSDS round

    • Arathar

      There are serval reason why verry slight spin stabilisation is usable and advantageous for flechettes in rifles. A shotgun with a bunch of flechettes put forward and rearward is a bad idea. They will fly sideways at closer range, and then with theyr low weight they wont penetrate, because shotguns cant handle high chamber pressures velocity is so low.

      Flechettes have to be shot straight and with high speeds to be efficient in any way. From handheld firearms.

      And APDSFS is a Tungstencarbide /depleted Urainium , singe Kinetic Energy Penetrator. Absolutly not comperable to shotgun flechettes.

  • manofsan

    As you say, since the flechette has greater accuracy at longer distances, perhaps it could become a good choice for a sniper weapon. A sniper doesn’t necessarily have to completely kill the target – sometimes it’s better to wound or incapacitate an enemy soldier to make them a burden on their fellow troopers, who’d have to carry the injured soldier back with them, thus taking more men out of the fight. If you’re fighting at close-quarters, you definitely want to be able to fully disable your opponents – a shotgun can be good for that. But against long-distance targets, your distance affords you more leeway as opposed to immediate need to immobilize the opponent. Besides, why couldn’t a flechette be made frangible, to inflict greater wound damage by breaking up inside the person’s body, instead of passing cleanly through? Also, poison-tipped could have significant effect. That narrower sonic cone that you were mentioning would perhaps make it more difficult to locate the shooter, which would be good for concealment, allowing the shooter to continue firing to inflict more damage on the enemy. As someone here said, anti-materiel rifle would be the best, since that’s also typically a long-range engagement weapon, and hitting these more solid targets means the flechettes won’t pass through as easily as they do for flesh.