Marines order polymer ammo

If Shell Shock Technologies ever gets going the way they say they are going to (making a bunch of new caliber cases), then I would presume the military would go with their stainless / nickel cases instead whereas they offer the same weight reduction, but are also reloadable, and are much more durable to shock and high heat - more so than brass even. Magnetic pickup is a quirky added bonus.

Military doesn’t want reloadability so much as weight savings and cost I would figure.

The polymer resists transfer of heat to the chamber and consequently the chamber remains cooler than with steel, or brass cases. Additionally the polymer case does not carry away the same amount of heat as brass when ejected. I have heard it said that; ‘10-15%,’ of the heat generated can be ejected with a brass, or metallic case.

However; the heat generated by the propellant has to go somewhere and with polymer based cases the heat, ‘apparently,’ goes up the barrel.

I watched the video and did not hear of any mention of the; ‘maximum sustainable number of rounds per minute,’ that can be achieved, and if this number is lower than that achievable with a metallic cartridge case.

The maximum number of; ‘sustainable rounds per minute,’ should not be confused with the cyclic rate.

Perhaps this polymer adaptation works well with the .50 cal. because of the availability of a Stellite barrel sleeve?

Stellite Uses


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How much heat is transferred from the inside of a brass case to the outside within a milli second or two?
That is the time a case spends in the chamber of a semi/full auto weapon. They always extract at firing (thats how autos work).
As you said metal cases is a good way of excavating/extracting heat from the system. Not the other way around.

I understand a plastic case will also cause higher temperatures because less heat is transmitted to the case “inside” upon firing. Means more heat is leaving the case mouth and will cause a higher temperature in the forcing cone from where the heat will move back to the chamber material.
Or is my assumption incorrect?

Not necessarily so. In fact a more efficient transfer of energy is achieved. Because less energy (heat) is transferred to the spend case and chamber, but instead transferred to the projectile and barrel, less powder is needed to propel the projectile.

The problem with heat does not come from the propellant burning in the case. Heat is generated by friction of the bullet going up the bore. In a full-auto weapon the heat transfers back to the receiver just as fast with brass or aluminum or plastic cases.

Plastic cases are good insulators but heat is generated by bullet friction.

Try this, shoot one round out of a cold barrel, you cat pick it up right away with hardly any felt heat, Then run 20 rnds semi-auto as fast as you can pull the trigger, then pick up the last ejected round. They all had the same amount or powder in them, so why was one hot & the other not.

Its a good point and you would think that with such a short residence time of the cartridge case in the chamber the opportunity for the heat transfer would be extremely limited.
I have tried to look up the science of this supposition.

It would appear that the answer is dependent upon the process of transient conduction and the temperature differential between the inside and outside of the cartridge case at the moment of maximum temperature (detonation).
The amount of heat transferred would be a function of the residence time of the hot cartridge case in the chamber.
Additionally; copper/brass has high thermal conductivity.

Some links at:

Instinctively; I believe that heat is conducted by a metallic cartridge case, to the chamber, prior to ejection.
A simple test might be to observe the chamber with a thermal camera, immediately after ejection of a case.

Would appreciate any thoughts.

Sam3

I have attended presentations by the founder of MAC LLC, who developed these hybrid polymer/metal .50 cal cases. He said that after firing 100 polymer rounds on full auto from a .50 BMG, he could put his finger in the chamber without getting burned. Not something to try with brass cased ammo…

Obviously, in continuous firing heat will gradually spread by conduction from the barrel to the chamber.

The link posted by “jestertoo” refers to MAC LLC. Note that MAC LLC is located in Bay St. Louis, Mississippi and NAMMO took over MAC LLC in 2018.

Brian