The Pros and Cons of Polymer Cases.


I’ve been talking with some folks about Polymer Cased Ammunition… and I found myself defending it.   Which is interesting because for all the bonus points polymer cased ammo gives you, there is one distinct drawback I’ll get to in a bit.

Polymer cases give us some advantages.  First off, is weight.  Now I’ve read some things that say you can save as much as 60%, but I think those numbers are either bogus, or maybe only in specific calibers… I don’t know.   But a more accurate figure I believe is the 40% reduction in cartridge weight.  The other big advantage is in cost.  Brass is expensive and the cost of that isn’t going back down anytime soon.  Or ever.  Brass requires a lot of work to make and shape.  Brass can sometimes get short in supply, and the commodities market can fluctuate the prices.  A polymer case can smooth all of those problems out.

As a former Light Infantryman, I can tell you… a 40% reduction in our combat load – that’s amazing.  That alone makes it work considering. That means we could be 40% lighter… which is nice.  Or we could carry 40% more.  Which is better.

Some folks question the strength of the cases, at the case heads and rims with a pure polymer case.   Valid concerns.  An Extractor could more easily tear through the rim instead of pulling the case out.   On the other hand, a polymer case would be less likely to be stuck in the chamber and thus would always be easier to get extracted than a hot expanded brass case that’s now fire-formed to the chamber.

An alternative to the polymer case head – is to use a polymer case body, but with a Brass, Steel, or Aluminum case head.   A two part case has been proven to be very reliable in Shotguns.   Because almost all shotgun shells are using a two part case.  How often do you ever see or even hear about a case head separation in shotgun shells?   Even back when the hulls were paper, you didn’t hear of it.  Ever.  You just didn’t.

Some guys argue that rifle and pistol cartridges operate under much higher pressures than shotgun, so this would remain a bad idea. Those guys don’t understand the Internal Ballistics.  See, all that pressure isn’t contained in the case.   It’s  contained in the Chamber.  See, we even have caseless ammunition and that stuff works without exploding the action.

Manually cycled repeaters will have had all the internal pressure dropped off to nothing before you open the chamber to cycle the action.   Automatic firearms all have mechanisms built in to allow the reduction in chamber pressure before the action starts to open.  This is for the most part, a non-issue.

We see case failures mainly because of a couple reasons.  One being incorrect headspacing in the firearm.  And two, cases that had been reloaded too many times at high pressure, causing the metal to become brittle, or thinned out due to metal flowing.   A rifle case gets longer when it’s fired.  This is why Reloaders have to use something called a Case Trimmer… Metal flows, gets trimmed, case walls get thinner.   This is a Material Science issue that Polymer could resolve.  Polymer is elastic so it can move, and then return to shape in exactly the same way metal doesn’t.

No, those are not the problems with polymer cased ammunition… The real problem with polymer cased ammunition is heat.  But not for the reasons your thinking.   You’re thinking a plastic cased round could melt into the chamber and cause a serious stoppage.  That’s not the problem.   Let me explain this… We’ve got a thermal dynamic issue here.   When a cartridge is fired, there is more than just pressure that is created.  There is heat.   A lot of heat.  The brass case will absorb some of that heat, and some of it will be conducted into the chamber walls.  And then upon extraction, some of that heat is pulled back out of the chamber and ejected with the empty case.

With a polymer case, the heat is insulated from the chamber.  At first, we might think that’s a good thing.  Keeps the chamber cooler.  At first it does.  But remember, energy can not be created or destroyed… it can only be contained or directed.  And with an ammunition cartridge being fired – it’s obviously directed.  This puts all that thermal energy at the case mouth.  Here’s what’s going to happen… we’re going to have a problem.  Increased Throat Erosion is going to be a problem, exacerbated by differential heating.  Especially in M-16 or other MSR type guns which are often taken from stone cold, to red hot in the matter of a moment.  All that heat hits the throat first and then radiates back to the chamber.  Over time, this differential heating will cause problems.  Though these problems will not develop quickly… but I do foresee them developing over time.   Much like the point of impact shifts in Ruger Mini-14’s as those barrels heat.  And eventually there could be fracturing.   With brass cases, the chamber and bore will heat up more evenly and we don’t get any differential problems.

To counter these problems the firearms will need to be manufactured with high quality materials and will need to be made to high standards.   Many guns we use now, will have no problems.  But the cheaper guns will become even… cheaper.    So… don’t use a lot of polymer cased ammo in cheap guns.

Here’s the thing though… I think polymer cased ammunition is the way of the future.    It’s advantages far outweigh the minor disadvantage.  Again, the main advantages are reduced weight and cost.  And I think the most key advantage is cost.  Ammunition cost remain the #1 Stranglehold on the shooting sports.  Ammo is expensive and makes shooting an expensive hobby, thus limiting it.  None of us shoot as much as we want.  None of us are really shooting what we want.  Cost of ammunition effects our choices on calibers and the guns to shoot them.  In one particular article, a number of top instructors picked their favorite calibers and almost all of them mentioned ammunition cost as an important factor in their selection.   I wonder if their selections would have been different if the costs were different.  Hmmm.     And how different would our own choices be?    What guns in what calibers would you be shooting if ammunition costs were different?    How would this effect the gun industry as a whole?   I think it would help the whole industry.   But that’s just me.

17 thoughts on “The Pros and Cons of Polymer Cases.”

  1. This is just me but brass is my way to go. Ya see I’m a d 72 yr. olde confirmed Luddite stuck in the tail end of the 19th Century. My favorite rifles are single shots; a Ruger #1 in .308 Win,beccause: basically a rimless .30-40 Krag. A rolling block in .30-40 Krag just because, a baby rolling block in .22Hornet, and soon a tipup single shot in .25-35 WCF because: easy on the shoulder, lethal on smallish ungulates such as whitetail deer, Coast Blacktail deer, and antelope. Also cheap to shoot because they are easy on brass especially when fired with cast bullets . And sparing of propellant at cast projectile velocities.

    I realize that the 35 of us ole fasshynnde shotists aren’t gonna change much if anything at all. But that’s just us.

    Your defences of polymer cases are all winners. but I’m still sticking with brass, it’s a known technology. 100 ctgs. is a large amount for me and larger amounts are simply back up.

    Hell, I’ve even started handloading .22 rimfires.

    Gerry N.

  2. Polymer cased ammo is even more logical for “low volume” shooting.
    Hunting-specific rifles, for example.
    The rifle that, regardless of how nice and enjoyable it is to shoot is reserved for hunting season and has zero checked before the season and then is only fired into game during hunting season probably won’t be significanty affected by the heat issue.
    As an added benefit, the money the manufacturer saves on brass for the case could be used on better, more consistent projectiles in a given hunting round.

  3. About 10 years ago I remember the shop that I used to work in sold polymer cased .223 and it was a 2-part case (either brass or steel case head to just past the case web…I don’t remember the metal material) and it was priced right in the middle of cheap stuff & premium ammo and it was in line with the wholesale pricing. It was only available for a couple of months as the manufacturer pulled the product as there were a lot of problems with it. Now I don’t know why they didn’t start with a pistol caliber instead of a bottle necked rifle caliber, but I’m not an engineer so there could’ve been valid reasons. Possibly it has/had to do with case capacity as the polymer case has to be considerably thicker than a metal case and that could’ve messed with pressures being consistent and not spiking.

    There must be some reason why a large ammunition manufacturer hasn’t released a polymer cased product as I’m sure that many of them have tried (I’d be willing to bet that the government did a study at Lake City) and for what ever reason it never worked out enough that they were comfortable enough to release it to the public. Personally I don’t know why caseless ammo isn’t being revisited again as I believe the technology is much better than it was in the 80’s and early 90’s when a number of companies actually did the engineering and had test material to work with.

    1. Ya Seattle/Tacoma had a polymer case maker, “Amteh” by USAC 1476 Thorne Road Tac WA, composite cased ammo, it was alum based and composite plastic above at about the same time frame and it looked good, could be reloaded with the companies manufactured hand tools in .38 spl and a priming tool on a truck tail gate. Amtech was not around long but it burned bright, it was shown at the Vegas Plastic industry conventions two years running. My step dad brought me a bag of loaded ammo, six round boxes they were giving away as product samples. Those were the days. Still have a bunch of the ammo loaded but notice that it fractures a lot when fired and I don’t thinks its safe. Shelf life is a consideration with ammo these days and you can’t really get better then brass with todays tech available. Plan a some point to write an article about the company but finding info is hard for some reason…

      1. I remember that stuff. I actually still have some still laying around as a novelty item. Haven’t seen any more of it around in years – even at gun shows. I guess cartridge collectors have snapped it all up.

    2. The Trounds were pretty big for a .38 Special round and it didn’t really have any weight savings over a brass cased round. I read about a guy that collect(ed) the Dardick pistols and even tried to 3D print Trounds without success.

      1. The advantage of a tround was that it was mostly it’s own chamber. Weight was saved in the pistol itself.

        The case was made out of Celanese Fortrel polyester. I think something stronger than printable plastic needs to be used.

        1. I’d rather see more development of the Gyrojet than Trounds. Trounds really solve no problems or offer any advantages.
          Gyrojet’s on the other hand I think are the step forward with Caseless Ammunition.

  4. Got about 5K of plastic cased .38spl with an aluminum head. Problem is that far too often when the case is fired, it tears where it holds the projectile(160grFMJ) and then sticks in the cylinder and won’t extract. I’m in the process of breaking it all down to salvage the projectiles and powder. Also going to try reloading the cases with a light load and a HBLWC projectile. We shall see what we shall see.
    IIRC the stuff cost me less than a nickel a round so what the heck.

    1. Might depend on when the stuff was made emdfl, the first rounds used standard wad cutter rounds, the later and last and most made stuff used a special made projectile 158 grn RNFMJ with a projection below the base that snapped into the case to hold it since the case diameter itself was smaller than .38 caliber. This later stuff was made with a white plastic body and the early stuff had a black plastic body on the rounds…lastly the company advertised several different designs of projectile, still have some of the advertising wish I could post it…

      1. Yeah this is the dark cased stuff although it does have the small kicked-out internal ledge to stop the 158gr proj. The other thing I noticed is that the proj is running almost .360″ This might explain why the loaded rounds are very tight when inserted into several different cylinders.
        At this point I’m going to try resizing the proj down to .355 and using them as sub-sonic loads for my 9mm suppressed.

  5. Please email me re: Carolina Arms Group new 1911’s. I will be at PBR Mathews on Monday 12 September and would like to see the Privateer & Veteran Carry. Sorry to interrupt this comment stream, but PBR Contact does not respond.

  6. One of the Miltech shows on cable had plastic cased telescoping rounds. The emphasis was full auto fire in a carbine. There were no details on shelf life. The idea was a full auto advantage with plenty of ammo since it was lighter than metal cased rounds.
    Who figures we have enough folks spraying and praying.

  7. What’s cool with polymer is that with the rigtt composition you will be able to see a very easy, cheap and reliable means of producing cases that will be the future. You may even see a cottage industry of it where people with affordable 3d printers can just print their own cases at home to their specs. These printers may even be a standard part of home reloading at some point if polymer becomes the way to go. I totally support this idea.

  8. The military is the main driver of this technology, no doubt, and the reason polymer ammo seems to be getting fast-tracked. And so far all I’ve read are the good points of this “wonder-waffen” ammo, and nothing about possible failures. But from experience, and history, what this might mean is, it’ll probably be rushed into service before all the problems are worked out. How long will we have to wait before we hear of weapons jamming mid-firefight because nobody tested it enough and the new polymer ammo failed? Seems ripe for ripped, deformed, and damaged cases, and that’s only for starters, since literally nobody has tried to use plastic ammo in war before. Think i’m being overly pessimistic? Maybe, then again, who can forget the truly ridiculous, yet very REAL, Vietnam era M16 fiasco? The rifle was rushed into combat in Vietnam c. 1965-66 before the rifle was really ready. Troops in the field were issued early M16 rifles with unchromed bores and chambers, in a dirty, hot, humid jungle environment no less. This, combined with ammo loaded with the wrong powder (that when fired quickly fouled the weapon) caused the rifles to jam so badly that only a cleaning rod could unjam it (just long enough for it to jam only a few shots later). These problems with the M16 cost many soldiers and Marines their lives – and there’s no reason to think a similar debacle can’t happen again. And to compound the issue – the Army is doing serious testing on a new 6.8 caliber round, right now – and will no doubt, soon require brand new rifles (and probably a new SAW type machine gun too) to fire the new ammo! All these factors might prove to add up to the makings of the proverbial “perfect storm” of Army small arms…

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