XM8: Polymer Cartridge Casings

[benEzra]

I seem to remember that when HK was developing caseless ammo for the G11, they found out they underestimated the role of the brass case in "absorbing" and "removing" heat from the action when it was ejected.

That's because the entire HK case was combustible (nothing to eject), so the burn was occuring directly against the chamber walls. In other words, the chamber walls were seeing 1200 degree gas during the whole duration of the burn. With NON-combustible-cased ammo, the polymer will protect the chamber walls from heat exposure, so it's not the same situation.

 

[Badger Arms]

That's because the entire HK case was combustible (nothing to eject), so the burn was occuring directly against the chamber walls. In other words, the chamber walls were seeing 1200 degree gas during the whole duration of the burn. With NON-combustible-cased ammo, the polymer will protect the chamber walls from heat exposure, so it's not the same situation.

Exactly. The heat of combustion stays within the case and at best only moderately heats up the plastic whereas the brass is such a good conductor it heats up the chamber walls. But I don't think either the brass or the polymer stay in there long enough to significantly heat the chamber. I want to hear anybody tell me that their semi-auto has experienced a cook-off before I see the value of brass cases as cooling devices to me. In the meantime, I'll remain open to polymers. I think the military should stick with brass if for no other reason than the cook-off danger in MG's.

 

[PaladinX13]

artherd's got it right... basically, these poly cases are more effecient. Brass needs more grains of powder to create the same effect, which results in excess heat, and the need for brass as a heat sink. It's a case of the material providing a solution for a "problem" it's causing. Polymer cases circumvent the heat dump issue by not wasting excess heat in the first place.

If you read the AR link provided, there were heat trials run. Besides that, people with fancy degrees and livelihoods invested in making something that works.

 

[PaladinX13]

I think the military should stick with brass if for no other reason than the cook-off danger in MG's.

I'm confused... if the chamber/barrel is what is providing the heat to the unfired round. Then why would conductive brass be safer from cook-off than insulated polymer?

 

[Tony Williams]

You are right to be confused; the problem is that there are different effects here which are working in opposition to each other. It goes a bit like this:

When a polymer-cased cartridge (PCC) is fired, very little heat is transmitted directly to the chamber as its such a good insulator (a side benefit seems to be that combustion heat isn't wasted in conduction, so you need less propellant therefore producing less heat). However, the barrel gets just as hot as before, and over a period of firing the heat from the barrel will work its way back to heat up the chamber.

When a PCC is sitting in a hot chamber waiting to be fired, the propellant will heat up far less, so a cook-off is less likely.

The question is; does the action of a brass case in removing heat from the chamber cool it sufficiently to compensate for its greater conductivity which heats up the propellant more quickly while it's sitting in the chamber?

The pros and cons are likely to depend on the circumstances, but I would guess that most of the time the advantage lies with the PCC.

Incidentally, the makers do claim other advantages for the PCC apart from light weight. These include the fact that it doesn't 'dent' if mishandled, but returns to the proper shape.

Tony Williams: Military gun and ammunition website and Discussion forum

 

[MrAcheson]

Engineer here again.

The polymer case will insulate the powder charge from chamber heat one way and therefore reduce the likelihood of cook-off. The same effect will also insulate the chamber from combustion heat the other way. The powder may burn a little hotter and heat the barrel up more which will eventually transfer back, but its not something I'd worry about too much. Despite what some people may tell you, steel isn't an especially good thermal conductor compared to other metals like brass or aluminum.

Just because the polymer stays cooler does not mean that it is carrying away less heat. The specific heat of a polymer is much higher than brass. This means that it may take away the same amount of thermal energy but have a much lower temperature than brass.

The question is; does the action of a brass case in removing heat from the chamber cool it sufficiently to compensate for its greater conductivity which heats up the propellant more quickly while it's sitting in the chamber?

The only time a case cools the chamber (or removes heat from it) is when it is inserted cold before firing. A cold case entering the chamber may cool it slightly, but only slightly. Consider the mass of all that steel in the chamber vs. the mass of the brass. Even if the brass had more specific heat (which it doesn't), it is not going to cool the chamber much.

The hot ejected case doesn't "cool" the chamber. It simply gets rid of hot material before it can finish transferring its heat to the chamber in the first place. The hot case heats the chamber up, just not as much as if it was left in place. Whenever the gun fires the chamber is going to end up a little hotter. It has to because of conservation of energy. The question is which heats up faster. I'm betting the brass will heat the gun up faster so the plastic will be superior.

 

[PaladinX13]

That's how I understood it, regardless, even with a heated chamber it seems to perform fine... in the AR15 link, they heated up the chamber with three 30 round mags fired at full auto first, then tried the PCC rounds (both letting it sit in the chamber and firing it). There was no melting or noticable issues.

 

[Badger Arms]

But then we come back to the tolerance of the polymer for heat. Let's say you just lit off a few magazines from your M4 carbine and you chamber a round and let it sit. Will the heat that will naturally transfer damage the case? The most heat is generated in the throat area of the bore where the bullet is being cut and pressures are at their greatest. This heat will naturally transfer to the chamber area eventually even if the case insulates the chamber as we are assuming now. Ignoring cook-offs for a moment, how much heat will the polymer take? Will the inevitable heating lead to the case sticking in the chamber?

 

[PaladinX13]

Badger Arms, you must have missed my post right above yours. In the AR15 link, forumites testing out the PCC rounds heated up a chamber with 3 30 round magazines of NORMAL brass cased ammo first (simply because it's less exotic than the PCC stuff they were trying out at the time) then put PCC round into the hot chamber to test for melting/sticking/off performance. The PCC ran fine even after being put in "brass heat" conditions (which it normally wouldn't experience anyways).

 

[Badger Arms]

You're right, I started replying before you posted.

 

[ShaiVong]

If anyone has an IR thermometer and some of that polymer ammo, try a magazine rapid fire from ambient and check the chamber temp, and outside barrel temp.. compaire with brass ammo.

I would do it, but i dont have an IR thermometer

 

[The_Antibubba]

Powder or primer cookoff?

When cookoff occurs, which component is combusting first? Remember, too, that the PCA rounds are not completely polymer-the base and primer pocket are brass.

The brass would certainly expand to the chamber, but surely the polymer won't, right? brass is quite malleable, but polymer doesn't necessarily hold a new shape unless it softens from heat. And since polymer is a poor conductor of heat, it isn't likely to heat evenly, like brass does. Hmm. Case ejection would probably result in separation of brass and polymer components.

And since I'm now totally confused, is this polymer really just a "plastic" or is it something else?

 

[MrAcheson]

The brass would certainly expand to the chamber, but surely the polymer won't, right?

I can only think of one material that doesn't expand when heated and thats a specific kind of graphite. If you heat and pressurize the polymer it will expand and seal the chamber. The question is will it return to its previous shape afterwards and will the case come apart? I'm betting no to the first and maybe to the second.

 

[Vern Humphrey]

Guys, a given charge of powder will only produce so much heat.

When the case is ejected, any head contained in the case is removed from the barrel -- leaving less heat.

Now a polymer case may be said to "insulate the chamber," but only momentarily -- because heat farther forward in the barred will be conducted back to the chamber area.

Over a series of rapid shots, a rifle or machinegun with brass cases will have less heat in the barrel than one with polymer cases.

 

[Badger Arms]

We keep going back and forth on this. The heat isn't contained in the case walls because it goes down the bore with the other heat! Since the brass is a VERY good conductor of heat, it is heated by the discharge much more than a plastic case would be. Ever held a shotgun shell after firing? Not very hot. I think that since you know brass is VERY hot when it comes out it is very natural to assume that it is somehow SUCKING heat out of the chamber when the opposite is true; the cartridge is the source of the heat we feel on the brass.

There is a very simple test for this: Fire a cartridge out of a cold barrel and measure its temperature. Next, fire three magazines rapidly and let one shell dwell in the chamber for a minute or so and fire it. If your theory is true, the second case should be substantially hotter than the first. I don't think it will be, by any significant measure. The energy it would take to heat a brass case 50 degrees, for instance, is miniscule. The brass is very thin in comparrison to the chamber walls as was mentioned earlier.

You could do the 'griddle' test. That is, see how long you can hold the fired cases instead of finding an accurate thermometer!

 

[Third_Rail]

Badger, what I think we're going back and forth on is not that, but where the extra heat goes if not into brass/the chamber when firing a polymer round. I think it'd just heat the barrel more and creep back into the chamber anyway!

But in any case, does the fact that a company can make polymer cheaply mean that we might see some current production decent quality ammunition in obscure calibers? Then I'd be all for polymer.

 

[MrAcheson]

Now a polymer case may be said to "insulate the chamber," but only momentarily -- because heat farther forward in the barred will be conducted back to the chamber area.

Yes but this happens whether the case is plastic or metallic. Therefore it is a constant and can be removed from consideration.

Over a series of rapid shots, a rifle or machinegun with brass cases will have less heat in the barrel than one with polymer cases.

This has not been proven. The hot case leaves the gun in both situations. The brass case is hotter than plastic but not necessarily because it "removed more heat." Plastic has greater specific heat than brass and will remain cooler when heated.

Why did the chamber get hot in the first place? Well the first reason is heat diffusing down from the barrel and chamber throat. But this is the same for both case types so it doesn't matter. The second is the heat of powder combustion diffusing through the case before it is ejected. The plastic case is better with this because it insulates more. The last is the heat absorbed by the new cold case entering the chamber. This is both trivial in effect and not-desireable for reasons of powder/primer cook-off so it doesn't matter either.

 

[Vern Humphrey]

Quote:
-----------------------------------------------------
The heat isn't contained in the case walls because it goes down the bore with the other heat!
-----------------------------------------------------

If that is true then the ejected brass case will not be hot. (I assure you, it will be.) Also the barrel will not get hot if all the heat goes down the bore with the other heat. But we all know that barrels DO get hot. And no matter WHERE in the barrel it gets hot, the heat quickly spreads to the rest of the barrel through conduction.

Quote:
----------------------------------------------------
Fire a cartridge out of a cold barrel and measure its temperature. Next, fire three magazines rapidly and let one shell dwell in the chamber for a minute or so and fire it. If your theory is true, the second case should be substantially hotter than the first.
-----------------------------------------------------

Okay -- try it with a semi-auto that fires from a closed bolt. Shoot until the barrel is glowing red. Leave an unfred round in the chamber (and keep the weapon pointed in a safe direction) and see what happens.

"Cookoffs" are well documented -- clearly a round that cooks off in the chamber is hotter than one that doesn't.

Quote:
------------------------------------------------------
The hot case leaves the gun in both situations. The brass case is hotter than plastic but not necessarily because it "removed more heat." Plastic has greater specific heat than brass and will remain cooler when heated.
------------------------------------------------------

That is possible, but the insulating properties of the polymer also must be taken into ac????. The heat from the inside of the polymer will not quickly be conducted through the mass of the case, and hence IN THE TIME AVAILABLE, the polymer case will not have nearly as much heat as the brass case.

Now, if you "soaked" the polymer case in an electric furnace for an hour or so and got it hot through and through, it might well hold a lot more heat.

 

[PaladinX13]

Guys, a given charge of powder will only produce so much heat.

Which is a key point you're missing... PCC rounds run on less powder and create less heat to begin with (which is why they have a smaller muzzle flash as well), so a hot chamber is less of an issue. Arguably, if a soldier is running though more than 240 rounds on full auto (it takes about 8 30rd mags of PCC full auto to generate the equivalent heat of 3 mags of standard) he deserves to have his round cook off, but it's a non-issue since the PCC rounds have survived even a chamber heated to brass temps.

 

[Vern Humphrey]

Now, there you may have a point -- polymer cases may indeed survive a hot chamber without cooking off better than brass cases.

More experience with them will tell.

 

[Grump]

Folks, since I remember getting frozen foods on their own PLASTIC plates in 1979, which were rated to 370 degree or so ovens, I believe we are long on the road to having plastic-cased ammo that won't melt before the cookoff point is reached.

Anybody know what the cook-off temperature range is?? The '60s-vintage tests I recall had it at somewhere around 325 degrees. An old American Rifleman was debunking a story about kids lighting off rounds by dropping them into a pot of boiling water (the were covering up for a minor accident they got caught in).

Those plastic plates were hard, but I'm entirely confident that flexible stuff can now be made to the same temperature performance standards.

I find it very believable that the part-plastic cased ammo won't melt into the chambers at temperatures below cookoff.

it takes about 8 30rd mags of PCC full auto to generate the equivalent heat of 3 mags of standard

That data is the most telling. If this is a report of heat generated to the barrel/chamber area, then we have proof of an advantage to the part-plastic cased ammo, far greater than the aluminum-case stuff the US Army experimented with during Vietnam.

I'd like about 20 of those fired cases for some measurements and bullet-seating expeiments. There's nothing like hard data to totally beat up the value of mere discussion.

 

[MrAcheson]

"Cookoffs" are well documented -- clearly a round that cooks off in the chamber is hotter than one that doesn't.

Yes but you are missing the point. 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 heat from the inside of the polymer will not quickly be conducted through the mass of the case, and hence IN THE TIME AVAILABLE, the polymer case will not have nearly as much heat as the brass case.

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.

Keep in mind that while steel will conduct heat, the barrel and chamber are not at uniform temperature. In current firearms, the chamber is warmer than the barrel. Making heat take the long trip up the barrel and back down to the chamber spreads the heat out more. Not as much heat gets back to the chamber because it is used warming the barrel. This creates a net reduction in chamber temperature. This is a good thing.

You need to realize that having the heat conduct quickly through the mass of the case is a large part of what heats the chamber up in the first place. The case gets hot but so does the chamber its connected to. With a plastic case the case doesn't get as hot so the chamber stays cooler and the heat has to go elsewhere before it can come back.

 

[Third_Rail]

Making heat take the long trip up the barrel and back down to the chamber spreads the heat out more.

Ah, but it also heats up the barrel faster, a problem if there was one. The rest of your statements seem good, as it does mean the barrel will act as a heatsink if you will for the chamber ever moreso with polymer cases.

 

[PaladinX13]

"Moreso" only in the sense of ratio NOT absolute values... because, again, there's less heat overall being created by PCC rounds. I can't really think of an apt analogy, but try this one:

If weight is conductivity and size is heat generated a PCC round is a sedan while a standard brass cased round is a bus. Likewise, the size of a parking space handles the size/heat of the vehicle/round.

Concerns about parking space in a lot adapted for buses, when driving a sedan, is ridiculous. Our guns have been adapted to tolerate "bus loads" of heat. So the sedan's impact on a parking space is much less than a bus.

Regarding ratio versus absolute values... imagine two people with the same lifestyles (bullet performance) but reporting their income taxes differently (reported income = heat). The PCC round reports, say $25,000 and gets taxed $20,000 by the Barrel, $2.00 by the case and $10.00 by the Chamber. The Brass reports, say $100,000 and ALSO gets taxed $20,000 by the Barrel and $20,000 by the case and $20,000 by the chamber. So you can say that the Barrel plays a more important role in taxing the PCC round than the Brass, which is true, but in terms of absolute values they PCC is still better off (less heat, same lifestyle). Bah... this is a terrible example.

The simpliest thing I can say is "See the results." I'm sure most people can think themselves into circles if they try but it's much harder to argue with results.

 

[Third_Rail]

I understand that they PCC causes less heat overall, but for short bouts of firing where brass can't catch up in terms of heat transfer, the PCC would heat the barrel more.