Future Firearms Ammunition Technology 002: Polymer-Cased Composite Ammunition – Lightening the Load, Pt. 2

So far, the polymer composite case has only found purchase with low-power specialty ammunition, such as the plastic blank and fired 7.62mm UTM marking round, both on the right. Several commercial composite cased rounds have been tried, including the grey .223 Remington PCA ammunition. In the 1970s, Frankford Arsenal and AAI experimented with composite cased ammunition, represented by the white cased round in the middle. On the left is a standard Korean-made M855 round.

In the last installment, we talked about the growing need throughout the 20th Century to reduce the weight of the cartridge case, to lighten the burden of the soldier. Experiments in aluminum have thus far proven unsuccessful, but another material is even more promising: Polymer. Plastics and polymers burst onto the scene in the post-war era, and it didn’t take very long for engineersto start looking at them as a way to reduce the cost and weight of ammunition. If feasible, polymer is an ideal solution for cartridge cases, as it is even less dense than aluminum, while being cheaper and using no metals or other expensive strategic resources, just crude oil.

Polymer does present some interesting challenges to ammunition engineers, however. Common polymers like Nylon possess a high strength-to-weight ratio, even compared to brass, but their stength-to-volume ratio is poor. This means that polymer case walls must be dimensionally different than their metallic counterparts, and that traditional case heads with extractor rims must be made of a different material. This combination of a polymer case body adhered to a traditional metallic base is called a “composite” case design, and is perhaps the most mature lightweight case design today, with a few such offerings on the commercial market in high pressure rifle calibers (although these have had mixed success). This basic concept has been in development since at least the 1970s, but recently a company named MAC, LLC has developed a successful composite aluminum case, and in the process taken out several patents on their design.

The most obvious material to use for the base of a composite case is brass, but steel is also easily used, and either gives a modest weight savings of about 15-18%. However, I think the best possible result would be an aluminum base. Careful design of the polymer case body and metallic base could allow for a successful, reliable cartridge case that solves the problems of an aluminum cased round while giving a substantial weight benefit versus either brass or steel based composite ammunition, rivaling that of the CTA concept which will be discussed later.

Whatever material is used, the composite case concept is mature enough that it seems lightweight cased ammunition is just around the corner one way or another, whether that’s via this concept or something more ambitious. However, we shall save the latter for another episode.

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 is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


  • Warren Ellis

    Is it possible to reload polymer cases like these like what can be done with brass or steel cartridges?

    • mig1nc

      People reload shotgun shells which are brass or aluminum rim with plastic hulls.

      • mig1nc

        The hulls are toast, but you keep the rim from what I understand. I don’t reload myself, but know folks who do.

        • chris lynch

          Pretty sure you can reload plastic hulls a few times with most normal sporting loads.

          • ostiariusalpha

            The typical load for a 12 ga. operates at under 10,000 PSI, whereas the typical .308 Win or .223 Rem operates at over 50,0000 PSI. This wouldn’t be an insurmountable problem, but modern rifle cartridges are usually necked and there is no known method to resize the neck on a polymer case, meaning that you can’t seat another bullet. Even swapping out the plastic hull is problematic, since the easier it is to detach and replace the hull manually, the more likely it is that it will also detach on you when fired at that +50,000 PSI. Which is something that Alex C. and Patrick R. found out when they tested some from PCP Ammo back in 2014.

          • FarmerB

            Oh yes, I was doing it 30 plus years ago for 12 g shotgun (and paper before that)

        • Ken

          The whole hull is reloadable until the petals crack. Even then, they could be reloaded if you really had to in an emergency. You can seal the top of a shotgun shell with wax, glue, tight fitting cardboard, or lightly fusing it together with a heated nail.

      • marathag

        And you really don’t need the end to be all metal, had great luck with the old Activ all plastic hulls in the ’80s, and they reloaded ok

  • Geoff Timm

    What about the classic expansion contraction problems when you combine dissimilar materials? Open the conex and discover all your ammo is now separated into bullet, case, base and loose powder. Geoff Who hopes military testing takes extremes of temperature into account. But I have little hope after the AR-15 Sabotage and murder of our troops in Viet Nam by our own officers.

  • Pete Sheppard

    What about heat tolerance and cook-off resistance?

    • ostiariusalpha

      Nylon and other polymers are more resistant to cook-off than metallic casing materials. They act as an insulator, keeping heat from transferring from the interior of the case to the chamber, and also the reverse. The problem is, what heat transfer is taking place in the metallic case head?

    • Depends on the design of the specific ammunition, but the concept has had problems with it before.

      • Pete Sheppard

        I remember seeing some polymer cases a while back. It’s hard for me to imagine polymers NOT melting or otherwise having problems in the heat of a fired-up chamber. :/

        • “Polymer” is a very broad term, sort of like saying “metal”. Some polymers would melt under heat, but some are pretty heat-resistant. The latter are the kind you’d use for this application, of course.

    • Kivaari

      What about melting in the chambers. As hot as chambers get, enough to cause cook offs, there must be some really special plastic that wont degrade and become a glob of goo.

  • Chipsa

    The caption appears to be wrong: ” such as the plastic blank and fired 7.62mm UTM marking round, both on the left.” Should probably read “on the right”

    • ostiariusalpha

      LOL! Yes, you pass our secret leftsy-rightsy test. Well done!?

    • Fixed.

  • flyfishr

    Polymer cases will not work in a firearm with a fluted chamber. That would have been worth mentioning in the article.

    • Kyle

      Is there anything in our inventory that is still issued with a fluted chamber? That’s a problem for Europe and they are mostly replacing those weapons as well. The FAMAS was the last big hold out and the French are replacing that soon.

      • Out of the Blue

        There’s the MP5, but that’s getting close to retirement.

        • Kyle

          Never saw one MP5 leave the armory. We had racks of them that just sat there and got eye f****d by all the lance corporals drawing M16A2s.

      • Blake

        Anybody still using H&K G3 rifles?

        • Turkey and Germany immediately come to mind.

          • Kyle

            I was under the impression that the G3s were all slowly getting phased out for 417s.

          • Both countries are still using G3s for the time being.

      • flyfishr

        Whatever the military does, the civilian market trends to follow. Cause….. mil spec is the best, even when it’s not.

  • Kivaari

    The only composite ammo I used was .38 Special years ago. It was desogned to be reloaded easily suing proprietary components. But it had issues. It used an aluminum case head, and I distrust those, just like CCI Blazer and the white plastic they used failed letting chunks of casing gone missing. That same combination would not be enough to give me faith in any round constructed using the same material.

  • Tim

    Composite cases sound just like shotgun shells…

    • Blake

      …which is why max case pressure of a 3.5″ magnum 12ga load is 14000 PSI, which gets a slug up to about 1800fps out of a 28″ hunting barrel after burning up a lot of sloooow powder over a long period of time. This is why even with 12ga magnum slugs, shotgun recoil feels more like a long push than the sharp crack of a high-powered rifle (esp. when firing, say, .308 from a lightweight carbine).

  • Charles Applegate

    “no . . . expensive strategic resources, just crude oil.”

    Gosh, nobody EVER went to war over strategic resources like crude oil.

    • Crude oil is far more plentiful than copper or iron, and therefore much more suitable for production of ammunition casings.

  • Nicholas Trueblood

    shotgun shells have used polymer for years. might work

  • PK

    I never see it mentioned, but there’s a relatively well-known way around the need for two-part bodies (rim/case head and case body)… the extractor groove is inside instead of outside, and the bolt has a nose which snaps inside that. It provides good case support, great hold on the case, and helps with that aspect of the problem… but introduces further dimension issues by adding length. It’s a trade-off.

  • Elliot Lee

    Well it’s worked flawlessly in shotgun cartridges for donkeys years, and not just in single and double barreled shotguns, but pump action, semi auto and full auto as well.
    So it’s obviously achievable.