XM8: Polymer Cartridge Casings

[JWH]

OK, let me see if I've got this. The main concern is that a PCC round will melt in a hot chamber and gum up the works like a plastic bread bag melting against the side of a toaster. But any temperature that will melt a PCC will also melt brass. And, of course, that point is moot because the brass-case round will cook off long before it melts, and a PCC round will cook off slightly later, only because it's more insulated.

The bottom line is that PCC rounds seem to perform their basic task as well as brass. And though they can't be reloaded, they offer other advantages as mentioned by Natec's marketing information.

Real-world testing will bear out these theories one way or another, and we'll all learn something, and this just might represent a true advance.

John

 

[Geech]

It seems like the biggest benefits may be to the military rather than civilian gun owners, but I am also very interested in seeing how this turns out.

 

[Badger Arms]

It seems like the biggest benefits may be to the military rather than civilian gun owners, but I am also very interested in seeing how this turns out.

I disagree. Millions of rounds of Wolff crap ammo are sold each year. Don't you think people would buy cheap ammo that actually didn't destroy the guns it was intended to fire in? I'd imagine that plastic and brass have about zero chance of damaging a gun.

One question I do have is if these 'lower pressures' also result in reduced velocity.

 

[Geech]

The benefits of decreased weight, price and heat would be multiplied in the military. Your average gun owner will get some benefit from this, but I think the differences in weight certainly and perhaps heat will be much less significant than they will be for the military. That's not to say others won't experience any benefits.

 

[Badger Arms]

I meant price. I'd imagine you could sell these cheaper than ammo made in Russia. Don't know, but I'm pretty sure.

 

[Vern Humphrey]

Quote:
---------------------------------------------
A cold cartridge will cool the chamber slightly, but only until it is fired. The net heat imparted on the barrel/chamber by a round after a load-fire cycle will always be positive. This is the whole reason that the gun heats up in the first place. If you can reduce the amount of heat imparted into the gun (and specifically the chamber of the gun) by firing, it doesn't matter that the cold plastic case doesn't cool the gun as much on insertion. Also since the plastic case has higher specific heat it will actually cool the chamber more on insertion (it will simply take longer).
---------------------------------------------

The cartridge doesn't "cool" anything. Heat, not cold is a positive factor. The case absorbs heat and is removed from the gun, leaving less total heat in the system.

Quote:
-----------------------------------------------
Again you miss the point. In order for the heat of combustion to warm the chamber it has to go through the case into the metal of the chamber walls. Otherwise it is either used to warm the case or goes up the barrel warming it and the propellant gasses.
-------------------------------------------------

Heat obeys the famous (and badly misused) Second Law of Thermodynamics. Gun barrels, being mostly made of steel, facilitate this.

Given a fixed amount of heat (from a fixed amount of powder) the amount transferred to the system as a whole would be pretty close to constant.

That is, a gun fired once with the brass case left in the chamber would be as warm as if a polymer case was used and left in the chamber. But if both cases are immediately ejected, the amount of heat left in the gun would be reduced by the amount of heat contained in the ejected case.

 

[benEzra]

The case absorbs heat and is removed from the gun, leaving less total heat in the system.

This would be true only if the case were chambered, then later ejected without being fired. The amount of heat the case absorbs from the hot chamber is miniscule compared to the heat the case transfers TO the chamber during combustion and before being ejected. (Otherwise, prolonged strings of fire would cool the chamber instead of heating it, and cookoffs would be more likely with short strings of fire than with long ones.)

The function of the brass case as a heat sink for the chamber is so tiny as to be negligible. The brass is hot when ejected because it just contained 1200-degree gas, not because it absorbed heat from a 200-degree chamber.

Also, keep in mind that the main method by which the chamber cools is heat conduction down the barrel, into the receiver, etc., where it can be shed by radiation or convection.

 

[Vern Humphrey]

Quote:
---------------------------------------------------
This would be true only if the case were chambered, then later ejected without being fired. The amount of heat the case absorbs from the hot chamber is miniscule compared to the heat the case transfers TO the chamber during combustion and before being ejected.
---------------------------------------------------

No.

The case is not COOLING anything. It is removing SOME of the heat generated when IT was fired.

The heat involved is heat generated in THAT firing. Ejecting the case leaves less total heat in the system, so when the NEXT case is chambered, the barrel and chamber is not quite as hot as it MIGHT BE.

 

[otomik]

2002 International Infantry & Joint Services Small Arms Systems Section Symposium, power point presentation?

i think they should make a .38 special or .357 carry load with a lightweight glaser bullet and launch the product with some marketing tie-ins with smith and wesson or taurus and their titanium revolvers.

but this won't be the greatest cheap plinking ammo or anything when you factor in how much they'll charge the cheap foreign ammo companies like wolf, s&b, fiocchi for the patent rights.

 

[Geech]

The case is not COOLING anything. It is removing SOME of the heat generated when IT was fired.

And so does the polymer case.

 

[Vern Humphrey]

Yes, but just not as much.

The insulating qualities of the polymer work against it -- heat cannot migrate throughout the case in the time avalable, so it carries only a fraction of the heat load it might have.

 

[Badger Arms]

Once again, I'd like to see statistics.

 

[Vern Humphrey]

We'd have to have statistics both ways, wouldn't we?

How do we get statistics on polymer cases?

 

[Badger Arms]

First we need money. Then we need to hire a lab. Know any philanthropists.

 

[artherd]

We keep going back and forth on this.

Shrug, people went back and forth on wether the earth was round or flat too.

It wasn't flat.

(PS: Badger, this isn't directed at you. Rather at those in general who refuse to accept the physics laid out before them.)



The basic question is wether the polymer cartridges will leave remaining in the gun more total heat after being ejected, than brass.

Does the few grains of powder ( to produce the same mv now) reduction more than make up for the lack of tossing out a burning-hot casing?

Will the polymer infact actually remove more total thermal energy than brass even does, even after the few grain reduction in powder needed?


Bottom line: I too would like to see some hard data on the actual materials at hand here. But my smart money is on the gun overall running cooler with the Poly cases.

Civies will see the same benefits the military does. Untill the next LA riots, we may not fully appricate them.

 

[artherd]

 

[ShaiVong]

We dont even need to do tests; anyone want to do some calculus concerning the transfer of heat and the thermal coeffecient k?

very small dt, and solve for the specific heat of the material.

How long does a case stay in the chamber after firing? What temperature does the ignition generate? 1200F?

 

[Badger Arms]

Math is not one of my strengths. I'd hit an engineer up for the answer. It's not simple math, there's other factors involved.

 

[Ironbarr]

Whoa, Preacherman... hold on a bit...

if they haven't had "extensive" field testing, maybe we could make a deal with them for - let's say - 100 rds per AR owner, let's say?

I'm sure that we could muster enough shooters and enough geography and weather conditions to provide a reasonable feedback for them.

Of course, slippery as I am, I'll nominate you to head this up. And if you can negotiate an extra crate or two, we'd REALLY test them out.

Caveat: If those rounds break my AR they buy it for lab tests - or replace it, one.

Deal????

-Andy

 

[cordex]

As to actual testing, I picked up a few boxes at the last gun show and will be going shooting with a friend who is getting a case that he plans to burn through in a day or so between a few different ARs (my Frankenrifle included). The little bit that I've shot so far went bang well enough.

We don't have remote thermometers or advanced testing equipment, but I'll try to do a write-up on accuracy, velocity, reliability, weight and other issues when we actually get out and use this stuff.

 

[griz]

I shot some of it yesterday from a Mini-14. Unlike a brass case you can pick it up immediately after firing and the case is only slightly warm. I have no way to measure temperatures so have no info on barrel warming.

I didn't measure velocity but noticed its empties were ejected to a totally different point than regular brass. This may be because the case is so much lighter.

Two out of ten rounds had case splits at the neck / body junction. The splits were around the circumference, not along the length of the neck like you see on a well used brass case. Splits on the first firing rule out further use by me.

 

[Third_Rail]

That settles what I'll use it for, then.

Cheap plinking and that's it. The military can have it, and it makes sense, but for civi reloaders, it's not usable.

 

[WhoKnowsWho]

As an update, my free issue of Gun Tests had a write up on this stuff. Works fine, no problem, just not as accurate as the other stuff they tried.

 

[MrAcheson]

Gun Tests also said that the ammo wasn't reloadable and had very mild recoil despite giving good muzzle velocities. They also said it was around 20% lighter than standard brass.