Future Firearms Ammunition Technology 008: Plastic-Cased, Telescoped Ammunition – Lightening the Load, Pt. 4

Polymer-cased telescoped ammunition, left to right: .38 cal. Dardick Tround, 5.56x30 Hughes Lockless, 5.56x45 Steyr ACR flechette.

Polymer-cased telescoped ammunition, left to right: .38 cal. Dardick Tround, 5.56x30 Hughes Lockless, 5.56x45 Steyr ACR flechette.

Previously, we discussed different concepts for lightening the soldier’s load, including aluminum-composite-cased and caseless ammunition. Today we’re going to look at the weight-reducing concept that many believe is the horse to bet on when it comes to next-generation small arms ammunition, and that is the plastic-cased telescoped ammunition concept, often referred to as cased, telescoped ammunition (CTA).

Where plastic-cased composite ammunition addresses the problem of extracting a polymer cartridge case by incorporating a metallic rim or base, the CTA concept takes a page from the playbook of caseless ammunition and asks the question: Do we really need to be pulling cartridges out of the chamber? Instead, CTA weapons are designed to push spent casings out of the front or side of the chamber, using the incoming round as both extractor and ejector. This layout requires a straight cartridge of constant cross-section, which engineers have often coupled to a telescoped layout where the projectile is buried in the round’s propellant.

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The three rounds from the title image, seen from the top. You can see how the projectiles are buried in the case. Note the petal type sabot of the 5.56mm Steyr ACR.

 

This concept does eliminate the problem of rim tearing with polymer ammunition, but it presents other problems. To facilitate this kind of cycle, a CTA weapon needs to have a separate chamber that moves so that the front end of the chamber is opened for ejection, but this also means that, like a revolver, the gap between the chamber and the barrel is not sealed. CTA weapons can address this in a few ways, including fixed, expandable chamber sealing devices attached to the chamber or barrel, or by using a sliding lockless design, but the problem of high pressure gas leakage remains a big challenge for CTA weapons engineers. Further, telescoping ammunition has some serious problems with gas leakage in front of the bullet, since it is difficult to keep the propellant from out-racing the projectile and burning ahead of it. In the most recent CTA ammunition configuration, this problem has been circumvented simply by not fully telescoping the round, and keeping the propellant behind the ogive of the projectile.

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Some LSAT ammunition, from a 2011 shoot. Note the 7.62mm concept round, laying on the ammunition box, and that its propellant chamber does not extend past the cannelure of the bullet. Image source: kitup.military.com

 

However, where the concept holds the most promise is in weight reduction: the CTA ammunition developed for the US Army’s Lightweight Small Arms Technologies program have demonstrated weight reductions of 32-36%, a massive improvement versus the old brass cased paradigm. Also, since the projectile and primer are the only metallic components in CTA ammunition, a great deal of strategic material could be saved as well if CTA ammunition proves feasible.



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.


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  • Thamuze Ulfrsson

    *Sniff* Poor little Hughes Lockless, you’ll be missed.

  • AK

    Since the bolt has to open anyway to be able to insert a new cartridge, I think the engineers are thinking this a little too hard. Weight savings can be realized using the current concept with polymer casing. The other solution would be to make the polymer casing inert but readily combustible, so that it burns out the way, akin to say a “new school Dreyse” cartridge.

    • pbla4024

      Then you can only inject the liquid propellant on trigger squeeze. It works for combustion engines 🙂

      • AK

        No, you could possibly have it in the firing chamber ready for ignition beforehand as well, just as you have the powder ready in the case nowadays.

        • pbla4024

          Only on firing fuel injection would prevent the cook off, right?

          • ostiariusalpha

            Meh, polymer cased ammo doesn’t have as much of a problem with cook off as metallic cases do, not to mention versus caseless ammunition. The polymer case walls don’t allow much heat to transfer to the chamber, and because the chamber is separate from the barrel, it doesn’t get as much heat transfer there either. As for gas cutting, a small ring flap around the mouth of the cartridge that gets pushed out into the barrel throat would be more than adequate to create a respectable gas seal. And regarding combustion engine fuel injection, that works because the fuel is aerosolized, you wouldn’t be able to fill the propellant chamber fast enough for automatic fire on a gun with a liquid propellant.

          • FarmerB

            Your biggest problem would be the propellant boiling off. You’re right it would be injected as an aerosol and would need pressure to contain and compress it.

  • Did the Steyr ACR solve the issue of high pressure gas leakage?

    • ostiariusalpha

      Not really, it still had gas cutting issues. The ACR’s “solution” was to lower the gas pressure to minimize the damage. Revolvers can go for hundreds of thousands of rounds before the forcing cone and top strap are sufficiently damaged enough to cause concern, so the ACR followed the same view that a little gas cutting was tolerable.

    • I am not sure; from the patent drawings it looks like it might be sealed than some other moving chamber designs.

  • Isaac Newton

    I wish a company would release an affordable bolt action or single shot rifle (and ammo specs) using CTA so that the regular enthusiast could experiment.

    • Paul Epstein

      Modifying something like a Contender barrel to work seems like it would be a lot easier than making and selling an entire gun as long as the cartridges work with a standard firing pin (or a device which replaces it), and you have a large potential user base who already own the firearm portion.

      Key problem would be making ammunition available. The standard handloader probably doesn’t have the unique tools and components required for telescoped ammunition, and even if they were open to buying it, you’d still want some sort of factory ammunition for people who don’t handload.

  • PK

    Not folded cartridges… please, anything but those.

    • George

      Does your aversion extend to prismatic folded designs such as Hughes Lockless?

  • Jeremy

    Every night I pray to the ghost of Stoner that the LSAT program will pull through.

    • Only if they can drop a kilogram from their carbine design, hah.

      • Stan Darsh

        Are you referring to the RDECOM 6.5mm CT Carbine?

      • ostiariusalpha

        I don’t see why they don’t just adapt the dang Steyr’s action, that gun was only 7.12 lbs; and it’s not like it is covered by any patents after nearly 30 years. Change from the fixed firing pin to a hammer or striker fired mechanism so that you have closed-bolt precision, and leave the open-bolt option for full automatic fire. That would make for a very nice service rifle. You could even move the grip back a bit (and shorten the trigger linkage), which would leave some room for a adjustable stock with a sane LOP.

      • Umberto

        how heavy it is now?

  • Kyle

    You need to do an article on whatever gear is going to be added to the grunt’s load if this is implemented. Oh you guys just freed up 15 pounds of ammo off the grunts? Sounds like its time to find 15 extra pounds other gear to make them carry. Probably more E-SAPIs. mounted on the thighs or something.

    • noob

      ceramic codpiece

  • Tim

    I’m sure you covered this like seven episodes ago, but what is the weight distribution for a std 5.56 cartridge between bullet, propellant, and case/primer?
    Thanks.

    • 5.56 NATO values are approximately:

      4g (61.7 gr) bullet
      1.7g (26.2 gr) propellant
      6.05g (93.4 gr) case
      0.25g (3.86 gr) primer

      • Tim

        So the business parts of the cartridge are even slightly less weight than the case. I can see why so much effort is expended on this research. It could potentially double ammo load out.
        Pretty ingenious to have the spent polymer case/shell come out the muzzle with the next round.

        • With the LSAT/CTSAS type mechanism, the case actually is pushed out of the rotating chamber by the next round during loading, not firing. Sort of like if you imagine a single action revolver, using the ejector rod to push out a case, back-to-front, not front-to-back.

          • Tim

            Thank you. Based on my shotgun shell reloading experiences with different kinds of wads, it’s not altogether unfeasible to have a cellulose or polymer case (without the brass of course) compress and come out the muzzle. Some of the new power wads even act as a sabot to tighten up the shot pattern. This concept would be more similar to a rifled slug.

          • No problem, thanks for reading.

  • lostintranslation

    Nathaniel…..I’m just curious about your comment; ” since it is difficult to keep the propellant from out-racing the projectile and burning behind it.”

    If the propellant is out-racing the projectile would it be burning in front of it?

    • Yes, and that’s not good for the throat of the barrel.

      • lostintranslation

        Nathaniel…..could I tentatively suggest that, the Article text might be clearer if it were as follows: “since it is difficult to keep the propellant from out-racing the projectile and burning in front of it.”

        • You are absolutely correct! Thanks, I will fix that.

  • jerry young

    I just heard this, sometimes when you try to reinvent the wheel you come up with a better version most of the time you do not! all this caseless and plastic cased ammo seams to be taking us back to the muzzeloader days when the first made breachloading guns that had no cases and was loaded much the same as a muzzelloader only in reverse, I’ll stick with the tried and true modern ammo, caseless ammo my have a place in military applications such as aircraft mounted machineguns this could save a lot of weight but I don’t think I would have wanted to carry anything like this into combat!

    • Isaac Newton

      All things being equal the Army may be thinking along the lines: if a soldier could carry 30% more rounds, he is 30% less likely to run out of ammo in a fight, 30% less likely to have to affix his bayonet to fight the enemy, need 30% less ammo resupply, 30% etc… They may not be right in the big picture, but at least their CT work seems data driven.

      • Arathar

        Yes and that with less chamber heating, and more reliable mechanism. + way more easy to clear than a ripped off brass ejection grove endportion with the case stuck in the chamber.

      • jerry young

        Since when has the Army and the government got things right? really my main problems I have with this is without the case the round can become deformed by bits of propellant chipping away or rubbing off from friction and exposure to moisture and the elements, the primer would have to be like that of a 22 that would be susceptible to exposure and if struck accidentally could go off or a normal primer that is inset into the charge that has to be removed after firing somehow, I’d have to question keeping case less ammo loaded into a magazine that puts spring tension against the loaded rounds, all of this points to needing a case that protects the individual round which would replace the weight of the cases for the amount of ammo you carry not saving weight in any way and making reloading harder not to mention cleaning the gun more often because the chamber gets dirty way faster without the case to keep it clean, then there is the problem of live embers remaining in the chamber as another round is loaded much the same as in a muzzle loader, as far as the telescoped ammo it seems like the action needed would be to complicated and bordering on stepping back in time rather than forward, what seems to save this magical 30% may not always be the better solution but this is just my opinion formed in over 50 years around firearms from muzzle loaders to machine guns, I could be wrong you never know!

        • Isaac Newton

          Agreed. Skepticism is healthy. I was playing devil’s advocate.

          To play devil’s advocate more (sorry), a new autoloading CT action can use the incoming round as the extractor and ejector. This eliminates two moving parts. Perhaps a modern Stoner or Kalashnikov could design an action/magazine that is actually even simpler than its brass cased brother. Actually in one of Stoner’s patents (US4770098 A) for CT ammo he talks about SIMPLIFYING the action: “In addition to significantly reducing the combat weight of a gun mechanism, the gun mechanism may be made with fewer moving parts, which, in turn, offers increased reliability and facilitates maintainability thereof.” Since the number of autoloading actions for CT ammo is so limited its hard to tell if the action would be more complex. Remember, repeating actions that use brass cased ammo have had a long time to mature (150 yrs).

          • jerry young

            Well here,s another way to look at it, the CT action will have to revolve the spent chamber out of line with the barrel to allow the incoming round to eject it by pushing the spent round out the front of the chamber that brings in even more difficulties like timing, cylinder to barrel gap or over pushing the new case to eject the old causing the cylinder to jam this action would turn the CT action into a semi auto revolver, much more complicated than the use of a rimmed case and extractor, again reinvention of the wheel that might not be as good in practice as on paper, I have nothing against revolvers in fact my favorite gun I own is a 357 revolver I grew up learning to shoot with revolvers after the military I thought about becoming a police officer they used revolvers as their main sidearm where I live, which changed shortly there after, I chose another path in life but was involved with the Military police we did carry the 1911, it will be interesting to see how long this takes to develop or not!

  • John L.

    Nathaniel, several times you’ve made the point that one impetus to this line of research is to save strategic resources. I can see that, certainly, in light of using less brass or none at all, lighter projectiles, etc.

    However, all else equal, manufacturing plastic takes a somewhat higher level of technology than many metals, and making plastic also requires use of a strategic resource: petrochemicals. Plus, as a general rule, plastics are more complex to recycle than metals, so the “civilian-held reserve stockpile” isn’t as accessible. So, I would suggest switching to a plastic casing doesn’t so much save resources, as change the balance of which ones are required to make a round.

    • Arathar

      Did you ever seen the dozends of machines needed to make a bottlenecked brass cartridge? While you can mold tousands of polymer cartridges in one step.

      Its about weight and performance, and mainly about LIVES.

  • Wiliam Deitrick

    Hilary says reasoning with someone threatening you with gun would talk him out of it. I wish she would have the opportunity to test her theory.

    • Ron Cole

      It is bullet proof. Just ask bill.

  • Leigh Rich

    When i was at Rodman Lab in the 70’s the military had been trying to come out with these . Hughes Gun System was working on it.

  • georgesteele

    Why can’t the base be a primer, the first 80% be propellant, and the last 20% be a sabot? Constant diameter, chamber:barrel, high base surface area to bullet weight ratio to increase muzzle velocity, tiny waste if the sabot is plastic or low cost. The primer doesn’t HAVE to be ignited by impact; it could be electrically ignited – or by impact/piezo spark. If weatherproofing is an issue, a light saran-like coating encapsulating the primer and propellant could still be breached by a strong-enough piezo igniter. For that matter, if the piezo or other-source electrical charge was carried by a conventional firing pin, it could break through the saran to deliver the ignition shock. The next round would clear the chamber of residue when loaded. I’m sort of a sabot bigot, but apart from the challenge of selecting a sabot material that isn’t of low dimensional integrity, so that the bullet is chamber- and barrel-concentric, I would think this would be eminently retrofittable (not a word . . .) Hey, even a blind squirrel finds a nut once in a while; who knows? Nice article.