Why Plastic Cases Are a BAD Idea

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jski

jski

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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!
 
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jski said:
It carries away almost no heat energy when ejected,
Click to expand...
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!:)
 
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.
 
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.
 
mcb said:
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.
Click to expand...
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.”
 
higgite said:
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.”
Click to expand...
Your rifle throat votes against this being a good dea as the most important thermally attacked item.
 
mcb said:
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.
Click to expand...
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.
 
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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.
 
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higgite said:
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.”
Click to expand...
With as high a specific heat as brass has, I suspect it’s carrying away a sizable amount of the heat energy with it.
 
jmorris said:
That would be “another reason why…”.

Things like this View attachment 1032589

and poor neck tension with regular bullets are above heat transfer on my list.
Click to expand...

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.
 
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.
 
Slamfire said:
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

View attachment 1032616

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.

View attachment 1032617

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.

View attachment 1032618

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.

View attachment 1032619

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

View attachment 1032620

View attachment 1032621



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.

View attachment 1032622

almost popped!

View attachment 1032623

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

View attachment 1032624

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.

View attachment 1032625

An 80 kpsia cartridge has to be an accident waiting to happen. Things go wrong faster, and more often, at higher pressures.
Click to expand...
I believe True Velocity is using steel heads.
 
jski said:
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.
Click to expand...

jski said:
With as high a specific heat as brass has, I suspect it’s carrying away a sizable amount of the heat energy with it.
Click to expand...
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

A worrisome trait about the M134 is the way it can cook-off live rounds with brass-cased ammunition. A cook-off occurs when a cartridge sits in a hot chamber and spontaneously fires because the powder inside the case becomes hot enough to ignite. Dillon has a 1,500-round test they run on the Minigun and brass-cased ammunition will cook-off after sitting in the chamber anywhere from 3 to 60 seconds. Dillon recorded a 20-percent lower bore temperature using True Velocity ammunition (thanks to the case insulating the chamber), so they ran a 2,200-round test on the ammunition. Dillon had to wait 5 minutes before one case got hot enough to melt — but the bullet never left the barrel. The absence of cook-offs in a Minigun should bring a welcome sigh of relief from special operations soldiers and aviators alike.
Click to expand...
 
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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.
 
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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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