Why Plastic Cases Are a BAD Idea

Discussion in 'Handloading and Reloading' started by jski, Oct 19, 2021.

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  1. jski

    jski Member

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    The specific head of brass is 2X that of steel. That means that your brass case require 2X more heat energy to raise a unit mass 1 degree (e.g., Celsius). Brass and steel have have similar density, so a steel and brass case for the same cartridge would have roughly the same mass.

    So when a brass case is ejected from your action, carrying with it the heat energy from the fired round, it should be actually cooler than a steel case. But in actual practice, I believe that both are equally hot, meaning the brass case is carrying more heat away than the steel case.

    Brass is better here in that it ejects more heat energy away from the gun. What about plastic cases? They carry away almost no heat energy when ejected, letting the gun get very hot indeed. VERY BAD FOR THE GUN!
     
    Last edited: Oct 19, 2021
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  2. Mark_Mark

    Mark_Mark Member

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    who is making plastic cases and were can we protest them???
     
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  3. Demi-human

    Demi-human Member

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    Or, it insulated the firearm against chamber heat more effectively, and directs it down the barrel. The barrel receiving the same heat from combustion and bullet friction anyway.

    I think plastic cartridge cases are dumb!:thumbdown:
    Brass has class!:)
     
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  4. earlthegoat2

    earlthegoat2 Member

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    If guess the military can tell us how they affect their full autos since this where the theory would have the highest relevance.
     
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  5. jski

    jski Member

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  6. barnfrog

    barnfrog Member

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    Having only taken funny book physics in high school, I'm certainly no physician. But perhaps one needs to consider how much of the heat energy produced by firing a round is transferred into the case and therefore doesn't go into the barrel. If the brass and steel take up 20% and 10% respectively, (assuming you are correct about all that specific heat and density stuff), then using plastic might make a significant difference. But if brass and steel only absorb 1% or so of the total heat energy produced then plastic wouldn't seem to change much of the equation.
     
  7. Mark_Mark

    Mark_Mark Member

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    now do you FL size or just neck size that case?
     
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  8. mcb

    mcb Member

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    Brass is an excellent thermal conductor and allot of the thermal energy gets transferred to the chamber walls. Polymer cases are insulators and reduce the thermal transfer to the chamber walls.

    There are good reasons to be critical of polymer cases, heat transfer to the firearm is not one of them.
     
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  9. The Glockodile

    The Glockodile Member

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    Physicist, you mean?
     
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  10. Mark_Mark

    Mark_Mark Member

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    Real question is, should I use a heavy crimp or just remove the bell?!?!?!
     
  11. jmorris

    jmorris Member

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    That would be “another reason why…”.

    Things like this 9100F132-F5FA-47B7-B7F0-5F8AAEA2CE82.jpeg

    and poor neck tension with regular bullets are above heat transfer on my list.
     
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  12. higgite

    higgite Member

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    This ^^^

    From a 2018 article about polymer cased ammo at gunsandammo.com:
    https://www.gunsandammo.com/editorial/true-velocitys-new-polymer-cased-ammunition/247607

    “A polymer case is an insulator rather than a conductor. When the cartridge fires, the case contains the heat and pressure and directs it all down the barrel. Instead of super-heating brass that then heats the chamber around it, the only heat the entire barrel sees is what's found in the bore.”
     
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  13. AJC1

    AJC1 Member

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    Your rifle throat votes against this being a good dea as the most important thermally attacked item.
     
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  14. jski

    jski Member

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    Where do you think that heat goes? It doesn’t just simply disappear! If it isn’t leaving with the spent case then it remains in the gun. Most likely right down the barrel with hotter gases thereby heating it up more.
     
    Last edited: Oct 19, 2021
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  15. Ru4real

    Ru4real Member

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    It’s called innovation. Sometimes it works out, and sometimes it doesn’t. Check back in 20 years for the outcome of how the plastic case experiment worked out.
     
    Last edited: Oct 19, 2021
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  16. jski

    jski Member

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    With as high a specific heat as brass has, I suspect it’s carrying away a sizable amount of the heat energy with it.
     
  17. Slamfire

    Slamfire Member

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    The joint between plastic and brass has always been a source of unreliability. In an unlubricated case, the front of the case adheres to the chamber, and if there is any clearance between case base and bolt face, the case sidewalls have to stretch when pressure rises. For jointed cases that will cause the joint to fail.

    As shown in a previous picture, these had their problems

    6GtK8mf.jpg

    Here the pressure vessel part of the case is inside the polymer body. I don't know the thickness of the sidewall, but they must have figured out given 0.003 to 0.006 sidewall stretch, a means of preventing sidewall separation.

    3hoTsgk.jpg

    This was interesting, the front of the case relaxs so quickly off the chamber, that the mfgr claims it does not drag. They have not introduced a bottlenecked version, so I assume this only works for straight walled cartridges.

    YyTN0vC.jpg

    you can see soot and scratches on the case sidewalls from the case dragging against the chamber. This is one reason case to chamber friction is bad. It takes force to pull the case out, the case may fall off the bolt face causing a jam.

    YOlW7nc.jpg

    here, the flutes are designed to break the friction between the front of the case and the chamber. The rear is only a gas seal

    z2Qej31.jpg

    PB9SaEH.jpg



    There are those that believe that cases are not there to be a gas seal, but are there to "reduce bolt thrust". In their mind, the case is strong and the action is weak. If the case is not supported, it will rupture, brass is far weaker than steel, and the case sidewalls are very thin, and the pressure vessel part of the case has to be supported or it will rupture.

    FE6dz3L.jpg

    almost popped!

    iJ7HxGf.jpg

    They never address caseless ammunition. Just how much bolt thrust does this case reduce?

    kIB4NPc.jpg

    Caseless ammunition have both positive and negative aspects.

    Caseless Ammunition Small Arms - The Good, the Bad and the Ugly

    I read an article where the military is considering an 80,000 psia cartridge which is a variant of this O'Connor steel head, but without the threaded joint.

    dUua1pp.jpg

    An 80 kpsia cartridge has to be an accident waiting to happen. Things go wrong faster, and more often, at higher pressures.
     
  18. .455_Hunter

    .455_Hunter Member

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    The promoters of plastic cases as a positive for heat management need to go back to college thermodynamics class. All of the thermal energy previously vectored away the system by the ejected brass case and conduction out of the chamber region is now deposited into the throat and barrel.

    Sure- you might see a reduction in cook-off potential, but at a huge decrease in barrel life.
     
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  19. jski

    jski Member

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    I believe True Velocity is using steel heads.
     
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  20. barnfrog

    barnfrog Member

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    It's a joke I need to stop using, I guess. Nobody ever seems to think it's as funny as I do.
     
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  21. mcb

    mcb Member

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    It's not going with the case, the amount of heat it takes with it is pretty small. Do the math**.

    <rambling warning>
    If the propellent gasses heat a 308 Win brass case (~11 grams of brass, specific heat .38 J/g-K) from room temperature to 205 C (401 F) that would ~750 J of energy. The temperature is generously high most ejected 308 cases are probably cooler than that.

    We know 308 pushes a bullet to roughly 3600 Joules of muzzle energy. Some of the initial chemical energy also goes into the kinetic energy of the propellent gases (roughly 2500 J in 308)

    We also know only about 20-25% of the chemical energy gets converted to kinetic energy of the bullet. So we have roughly 16,000 J of chemical energy (assuming 22.5% efficiency) in the propellent before we pulled the trigger.

    The overwhelming majority of the rest of that chemical energy goes into heat; heating the case, the gasses, the barrel etc. So the bullet takes 3600 J of the initial chemical energy as kinetic energy, the propellent mass takes 2500 J as kinetic energy leaving nearly 10,000 J of energy that most of is wasted a heat energy and the case takes only 750 J, slightly less than 8% of the waste heat goes with the case.

    Polymers typically have higher specific heats than brass but specific heat is based on mass not volume. Making the assumption that they are using a form of High Density Polyethylene (Specific heat; Brass: 0.38 J/g-K, HDPE 1.9 J/g-K) then we can still expect the hypothetical HDPE case to take roughly 50% of the heat with it that brass does (assuming the same final temp). It likely takes significantly less as the cases come out very cool in the testing have seen.

    The problem is that brass has a thermal conductivity that is 200-500 times higher than most polymers so while it takes less than 8% of the waste energy with it when it ejects it does nothing to slow how much heat passes through the case into the chamber that is a considerable amount of surface area compared to the bore and is also the place where pressures and temperatures are the highest at peak pressure.

    ** Doing everything in SI units cause thermal calculations in US customary units are a pain in the butt with keeping track of units.
    <rambling done>

    The biggest proof is simply the testing done with polymer ammo. They have shown that for a given number of rounds fired barrel temperatures are lower using polymer cased due to the insulation the polymer case provides over the surface area of the chamber. One example documented below.

    https://www.gunsandammo.com/editorial/true-velocitys-new-polymer-cased-ammunition/247607

     
    Last edited: Oct 19, 2021
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  22. n2omike

    n2omike Member

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    I'm you wrote all that (and I didn't have to). Lol

    Yes, only a small portion of the propellant energy goes into heating the case. That small amount of heat not wasted heating the case can go into driving the bullet. With plastic cases, there would be a small velocity increase (all else being equal) and a lot of the rest of the excess heat will just be blown out the end of the barrel.

    How much heat is actually transferred to the chamber by a brass case in semi-auto? The brass is not in contact with the chamber for very long at all... so there's not a huge amount of time to transfer heat... hence the reason the casing is still hot when ejected. I don't think there's really a significant argument for chamber heat in a semi or full auto.

    But, let's mention chamber heat anyway. IF it would be cooler with plastic cased ammo, it being cooler would conduct heat away from the barrel. So, any extra heat not conducted by a brass case would mostly go into driving the bullet a little faster, and most of the rest would just get blown out the end of the barrel. Whatever excess heat the barrel absorbs could get partially conducted away by a cooler chamber.

    I still don't think plastic cases are a good idea at this point, but I don't believe heat in the gun is the downfall.
     
    Last edited: Oct 19, 2021
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  23. mdi

    mdi Member

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    Just a thought; I have seen videos of guns run full automatic until the barrels glow (and some melt and sag). In none of these destruction videos does the action or chamber get hot enough to make the steel glow, so most of the heat is directed down the barrel. If some member has an infrared thermometer perhaps they could measure temps of brass, vs steel cases or aluminum cases or even the Shell Shock two piece nickel alloy cases? That would be interesting. I saw the above video quite a while ago, but I have never seen one of the "plastic" cases or know anyone that has, has the Armed Services dropped the experiment?...
     
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  24. Mark_Mark

    Mark_Mark Member

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    this is not a Joke forum sir! back of the bus!
     
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  25. The Glockodile

    The Glockodile Member

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    Lolz :rofl:
     
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