XM8: Polymer Cartridge Casings


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starfuryzeta
March 9, 2004, 12:42 PM
http://www.strategypage.com//fyeo/howtomakewar/default.asp?target=htweap.htm

To further complicate matters, the army is also looking into what appears to be the first workable non-metallic cartridge case for rifle rounds. American form Natec, Inc, has developed a polymer based cartridge case that is easier, and cheaper, to make than the traditional brass ones, and weighs 25 percent less. This means that ammo would be about twenty percent lighter to carry. The lighter polymer cases also result in less propellant being needed, and thus less recoil. The Natec cartridges use the standard brass or aluminum base caps, primers, propellants and bullets. A round for the M-16 is available now, and a 7.62mm round will be available this Summer. The Natec cartridges are new developments and have not received much field use and testing yet.

What does the makeup of the case have to do with the amount of powder needed to propel the bullet?

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Third_Rail
March 9, 2004, 12:48 PM
I hope that flops, I'd hate to see manufacturers switch to ????ty cases that won't hold up just because the military switched.

Lightspeed
March 9, 2004, 12:48 PM
I've got some of these!
They work really well.
http://www.flyerwires.com/plastic223.jpg

benEzra
March 9, 2004, 12:57 PM
What does the makeup of the case have to do with the amount of powder needed to propel the bullet?
I can think of three possible reasons.

One--under reaction conditions of 1200 degrees fahrenheit and 50,000 psi, the polymer itself could be combustible. If this is true, only the case head would be ejected when the action cycles. (Lightspeed, does the polymer case burn up when fired, or is it ejected with the brass case head?)

Two--the polymer isn't combustible, but soaks up less heat from the burning gases than brass would, thereby leaving more energy in the combustion products to propel the bullet.

Three--the polymer serves as insulation between the combustion products and the chamber walls, reducing heat loss and thereby leaving more energy in the combustion products to propel the bullet.

Third_Rail
March 9, 2004, 01:17 PM
More than likely, it's the third. I still don't like the idea of polymer cases, though.

Lightspeed
March 9, 2004, 02:02 PM
Yeah, the case ejects just like a regular brass one.
It's not hot either, when it does, you can pick it right up. I guess this might have something to do with it having less powder. It shoots just like the Winchester white box ammo, and is just as accurate.

MLC
March 9, 2004, 03:07 PM
As if the millions of tons of other discard plastic littering the earth wasn't enough.

SodiumBenzoate
March 9, 2004, 04:14 PM
I hope that flops, I'd hate to see manufacturers switch to ????ty cases that won't hold up just because the military switched.

Yes, because polymer is so weak :rolleyes:

I believe one of Cavalry Arms' telescoping polymer buttstocks took a bullet and the bullet deflecting down into the table it was sitting on, and the buttstock still worked fine.

As if the millions of tons of other discard plastic littering the earth wasn't enough.

Metal isn't exactly that much better.

Master Blaster
March 9, 2004, 04:37 PM
Metal isn't exactly that much better.

But you can reload metal, and you can melt it down and reuse it to make new cases.

Can you reload the polymer case???????

Archie
March 9, 2004, 04:42 PM
What does the makeup of the case have to do with the amount of powder needed to propel the bullet?The polymer is also probably thicker than a conventional brass case. That means the inner burn chamber is smaller in volume. A small volume requires less propellant for the same pressure level.

I want to know if they can be reloaded without massive re-tooling on my part. Does the case mouth retain tension? What about crimp?

Third_Rail
March 9, 2004, 05:19 PM
Yes, because polymer is so weak

I never said it was, but try making a stock out of the same polymer as the cases are made of. There is more than one type, and I'd wager the cases are made from the least expensive type.

George Hill
March 9, 2004, 05:47 PM
Yeah, we all know how useless the idea is in shotgun shells.
:rolleyes:

If it means cheaper surplus ammunition on the marker - go for it. I'm not a reloader, so I don't care about that.

clubsoda22
March 9, 2004, 07:12 PM
the stuff will probably be so inexpensive when it turns up as surplus, you won't care about reloading it. Of the reviews i've read about this ammo, they have all been good. Different color plastic cases will also make it very easy to distinguish different loads.

Vern Humphrey
March 9, 2004, 07:22 PM
Not to be pickie, but sausatges have casINGS. Cartridges have cases.

clubsoda22
March 9, 2004, 07:24 PM
who are you talking to?

Vern Humphrey
March 9, 2004, 07:26 PM
Check the thread title:

XM8: Polymer Cartridge Casings

For a moment, I though it was all about making summer sausage.;)

clubsoda22
March 9, 2004, 07:27 PM
you are being pickie :D

Vern Humphrey
March 9, 2004, 07:39 PM
Reminds me of a Law and Order episode, where the bad guy was shooting people with a double-barrel shotgun.

The brilliant detectives decided that since there were no "casings" found at the scene, he had to be using a "breech loader."

Well, duh!;)

Third_Rail
March 9, 2004, 08:00 PM
Or... maybe a musket! Wow!

Right, anyway, I'm all for cheap surplus ammunition, but I'm also for reloading and custom match loading. I don't want to see brass cases go the way of brass hulls (i.e. really expensive and hard to get)

natedog
March 9, 2004, 08:15 PM
Just curious, I haven't really thought of this before, but why isn't brass standard for shotgun shells? Why has plastic (so far) been used only for shotguns and not rifles?

Third_Rail
March 9, 2004, 08:29 PM
Think of how BIG hulls for shotguns are. People don't like spending a lot of money on brass.
Now think of how tough brass is to work after a reloading. People don't like wasting a lot of time gluing wads over their shells, crimping the plastic is easier.
Now, think of how much easier it is to produce plastic hulls compared to brass.
Now, how about the average pressures of shotguns being fired? Not too much plastic is needed to make them safe...
It all boils down to saving money.

IMHO, these two aren't benefits when applied to rifle and pistol cases; I want to be able to reload, I want to have a nice firm crimp. For military use only, plastic is fine, but if the industry switches over and brass cases go away, I'll cry.

Vern Humphrey
March 9, 2004, 08:34 PM
Shotgun shells (and many early rifle shells) were made of paper with metalic heads. The advantages of paper were many -- low cost, ease of manufacture, and so on. The disadvantage was paper could swell in high humidity.

Brass shells require a different chamber -- since the interior diameter of a brass shell is larger than for a paper shell in the same gauge. The Army used all-brass shells in WWII in the tropics, but loading for guns originally chambered for paper shells was a probem.

The development of plastic shells overcame all the disadvantages of paper, and has proven an ideal solution.

If we were to go to all-brass shotgun shells, the interior dimension problem would affect performance, and we would gain nothing -- in fact, we'd lose in terms of cost, weather resistance and ruggedness.

Preacherman
March 9, 2004, 08:55 PM
Lightspeed, where did you get that ammo? Can the rest of us buy some? I'd like to put a few boxes through my rifles to get a feel for it, and see how well it works in various actions.

Lightspeed
March 10, 2004, 12:24 AM
I got mine from a friend who's a gun dealer, I don't know wherre it's for sale anywhere else.
I do have their web address, though=
www.natec-us.com/products.asp
Maybe you all can track some down from there!

Onslaught
March 10, 2004, 12:25 AM
I'm all for cheap surplus ammunition, but I'm also for reloading and custom match loading. I don't want to see brass cases go the way of brass hulls
So you don't want technology to move us forward and let the rest of us to have cheaper ammo just because it would inconvenience you? :scrutiny:
(before you get offended, it was a joke)

Besides, if polymer cases are cheaper, then you could just buy THOSE and reload them... Just think, if they're really that inexpensive, you'd never have to resize brass again! Plus, if they use less powder, you're using less powder to reload, thereby offsetting any small increase in cost of the case itself.
;)

The_Antibubba
March 10, 2004, 05:04 AM
Can you imagine the Maryland DOJ trying to match a polymer casing to a particular gun?
:evil:

pignock
March 10, 2004, 10:33 AM
No - the first cry from the antis will be:



":eek: :eek: :eek: This ammunition can pass through airport metal detectors and therefore must not be available for civilian sales!!!:eek: :eek: :eek:

Blain
March 10, 2004, 11:45 AM
Hmmmm, getting rid of reloading......

Seems to me like the perfect way to help constrict the publics capacity to be armed up to parity. Once there is no more reloadable ammo, all one would have to do is cut off the supply of surplus and commercial ammo. Bingo! Dissarmed populace.

griz
March 10, 2004, 01:03 PM
I bought some of the PCA ammo at the last gun show. The price was about he same as white box. The weight difference is very noticeable. I can see why a soldier carrying 200 rounds, or a helicopter carrying thousands, would prefer the plastic. I'll weigh some tonight and post the results tomorrow.

Badger Arms
March 10, 2004, 01:07 PM
So, somebody tell me why plastic (which is more elastic than brass) would be less reloadable? High-temperature plastics can easily withstand the temperatures we are talking. Seating the bullet should be a breeze and there doesn't need to be ANY resizing. Just decap, prime, charge, and seat with your hands!

Black Snowman
March 10, 2004, 01:18 PM
Well, after the heating and firing depending on the formulation of the plastic I don't see it being able to retain the tension nessisary to prevent bullet set-back or maybe even hold the round depending on how loose the chamber was. Now if the plastic were intended to be reloadable maybe it would "remember" it's origional dimentions and go back to shape, which would make reloading a snap. I'm not sure if this is possible and affordable with current material technology. Any chemical/mechanical/material/plastics engineers here have some clues?

Poodleshooter
March 10, 2004, 01:20 PM
Seems to me like the perfect way to help constrict the publics capacity to be armed up to parity. Once there is no more reloadable ammo, all one would have to do is cut off the supply of surplus and commercial ammo. Bingo! Dissarmed populace.

They can already do that. Cut off the supply of commercial smokeless propellant or primers, and we're all back to scrounging our own mercury fulminate caps, and maybe making cordite at best.
I don't think my AR15 will run too long on FFFg Goex

They're already limiting reloaders in most states. My home state for example is passing more restrictive fire codes this year, limiting my possession of powder and primers even more. Those new restrictions are barely appearing on the radar of even the most attentive gun rights organizations.
Reloading has a very small political support base.
:(

Regarding this polymer ammo, has anyone shot any? Any empty cases available for study? I'd be interested if it gets flame damaged and splits at the neck after repeated firings. I'd imagine case life to be somewhat reduced, provided that it is even reloadable.
Anyway, I have enough brass cases to last until I have great grandchildren.

Engineer
March 10, 2004, 04:07 PM
FWIW, there's a thread in the ammunition forum on AR15.com about this ammo: http://ar15.com/forums/topic.html?b=3&f=16&t=179377.

MrAcheson
March 10, 2004, 05:56 PM
Well, after the heating and firing depending on the formulation of the plastic I don't see it being able to retain the tension necessary to prevent bullet set-back or maybe even hold the round depending on how loose the chamber was. ... Any chemical/mechanical/material/plastics engineers here have some clues?

Hi mechanical/composites engineer here. Your initial analysis seems pretty good to me. I would expect the pressure of the round to make the case deform. I would expect the heat of the burn to cause some stress relief so that it doesn't come back to quite the same shape afterwards. Its quite possible the inner layer of the plastic would be partially effected by the combustion as well which may degrade its properties in one way or another.

You might be able to recycle the polymer and remanufacture the composite case using the same brass case head, but reloading like brass might be a little much to ask.

I could be wrong though since I am not involved with this project in any way, shape, or form.

JWH
March 10, 2004, 07:23 PM
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.

I'd want to be sure that plastic cases don't contribute to overheating and cook-offs.

John

P.S. Check out www.natec-us.com.

Vern Humphrey
March 10, 2004, 07:28 PM
I once had an ejected case from an old Remington Browning .22 fall into my boot top. Even though I was wearing socks, it left quite a blister.

Cases take a LOT of the heat with them when they are ejected. I would think semi-automatic or full-automatic military weapons would not function well under combat conditions with "plastic" cases.

Badger Arms
March 10, 2004, 07:35 PM
Vern, JWH:

You might be missing something here. Brass is a VERY good conductor of heat. That means it HEATS UP quickly and transfers that heat to the chamber if and only if the fired case remains in the chamber. Polymers are very poor conductors of heat and that heat tends to stay in the case and bore therefore I'd imagine that less heat will be transfered to the chamber than a brass case.

Of course, the flip-side is that a chambered round will absorb some heat from the chamber and take it away. I'm not entirely sure that the amount of heat it absorbs is significant. Perhaps plastic cases will be less likely to cook-off because of their insulative properties.

Vern Humphrey
March 10, 2004, 08:04 PM
That's our point, Badger.

When a brass case is ejected, it takes a lot of heat OUT of the gun, which means the gun runs cooler. A gun with plastic cases would heat up much faster than one with brass cases.

After all, the amount of heat (given identical powder charges) is the same with both cases. The plastic case cannot serve as a removeable heat sink.

In addition, in a closed breech system, an overly-hot gun and a chambered plastic cartridge could spell real trouble.

JWH
March 10, 2004, 08:20 PM
Natec's website says their ammo is "currently under assessment by the U.S. Army," so hopefully they'll get some solid science on how polymer cases deal with heat management.

I don't know the physics of brass vs. polymer in this issue, so it's conjecture on whether polymer insulates the action from heating up in the first place, or whether it exacerbates it by not removing some heat like brass.

We'll still have plenty of heat in the barrel from the burning gas and bullet friction. I read about glowing, red-hot machine-gun barrels defending against human wave attacks in the Korean War. At what temperature will a brass case "stick" in the action due to a very hot barrel vs. this new polymer? And at what temperature will it just cook off? We need the input of some experienced combat machine-gunners who also have Ph.D.'s in physics!

I'm all for technological advances that, all things considered, truly represent a benefit to the user. These new cartridges could even have some surprise benefits once they're really well wrung out, but we'll see. . . .

John

P.S. Some guys over at AR15.com have been playing with this stuff since January of this year. See http://ar15.com/forums/topic.html?b=3&f=16&t=179377&page=1. They've had two dramatic failures, but, overall, mostly positive results.

Vern Humphrey
March 10, 2004, 08:51 PM
We do know that -- when you eject a hot brass case, you're removing a substantial amount of heat from the gun.

Now, how would a plastic case "contain" an amount of heat equal to a brass case? If the heat isn't in the case walls, where is it?

Third_Rail
March 10, 2004, 09:28 PM
In the barrel.... I think.

I don't like the idea of polymer cases, still, even after considering everything. Call me old-fashioned, but I like blued steel, walnut, and brass. And I'm only 17!

Badger Arms
March 10, 2004, 09:30 PM
Vern:

Don't get me wrong, I'm not for or against plastic. I do believe that most of the chamber heat is caused by the brass in the first place. The heat transfer TO the chamber and FROM the chamber probably equals out. The brass also would absorb MOST of its heat from the combustion itself rather than from the chamber. That's my guess.

I think that plastic cases have merit even now as low-cost practice ammo. I remember qualifying with 22's, with plastic bulleted .223, and with M193 but never with M855. To be fair, the Qualification guns we shot were all slick-sided M16's with 1-in-12 twist barrels. The 22lr's and plastic bulleted guns were abismally unreliable and innacuracte respectively. I think a plastic cased low-velocity training round could be used in an unmodified gun with great accuracy at perhaps a much lower cost. My opinion. They could make the cases Blaze-Orange for easy cleanup and identification.

Vern Humphrey
March 10, 2004, 09:38 PM
Let's take this a step at a time.

1. The heat is caused by the burning powder.

2. For a given charge and type of powder, there is a given amount of heat (let's say X calories.) There cannot be more heat generated than that.

3. If anything is taken out of the barrel, the heat (call it Y calories) in that thing taken out is subtracted from the heat remaining in the barrel.

4. The barrel now has less heat, X-Y calories.

From this, it follows that the more heat carried by the ejected case, the less heat remaining in the barrel.

Now, a brass case will carry more heat than a plastic case, and therefore will leave less heat in the barrel -- significantly less.

It may be postulated that this heat isn't in the chamber, but the barrel is made of steel -- an excellent conductor -- and the tendency is for the heat to flow from hotter to cooler parts of the barrel (anyone who had grabbed a just-fired machine gun by the barrel can attest to that.)

Badger Arms
March 10, 2004, 09:52 PM
Hmmmm, then I'm not entirely sure that the brass is pulling that much heat out of the chamber. Firing a 22lr in a cold barrel, the casing comes out VERY hot IIRC. One trick I did with a Ruger 22 was to fire the gun sideways and catch the ejected casings in the air. I could only stand to hold them for an instant and then I'd toss them at my friend next to me on the firing line! The first round was no cooler than any other round. That was low-pressure round in a cold barrel-chamber.

I'm of the opinion that the brass case simply does a good job of COOLING THE IGNITION TEMPERATURE by sucking heat from the combustion in the chamber. I'd say that this factor has more to do with the brass heating than the theory that the brass is somehow acting as a cooling agent.

And since it heats up that fast, I'm sure it would be transferring some of that heat to the chamber as well.

Taking the AR-15 as an example, I can't recall the receiver getting hot around the barrel in spite of the fact that it was in constant contact with the barrel. I think the chamber temperature is temporarily spiked but without the friction of the bullet to heat it up more, it really doesn't get that hot in common use.

Sure, full-auto fire is bound to cause the throat to heat up rapidly and that heat combined with what heat is transferred from the brass to the chamber is significant. The one factor that might work against the brass-heat-sink theory is if SAW's with their open-bolt operation were significantly different in the cooling realm than are automatic rifles like the M-16 which operate from the closed bolt. I'd think that a case dwelling in the chamber is much more likely to draw heat from the barrel if that's a significant factor in the first place.

Being the selfless person that I am, I will volunteer to test this theory. Could somebody please send me an M249, an M4 Carbine, and about 50,000 rounds of M855? I'll supply the thermometers!
:D :) :confused: :uhoh: :neener:

Vern Humphrey
March 10, 2004, 10:50 PM
Quote:
-----------------------------------------------
I'm of the opinion that the brass case simply does a good job of COOLING THE IGNITION TEMPERATURE by sucking heat from the combustion in the chamber.
-----------------------------------------------

We're saying the same thing. Now, if it were a bolt action weapon, and you simply left the case in the chamber, there would be no beneficial effect.

But in an automatic or semi-automatic weapon, the case is ejected, WITH the heat that would migrate to the chamber wall.

Quote:
------------------------------------
The one factor that might work against the brass-heat-sink theory is if SAW's with their open-bolt operation were significantly different in the cooling realm than are automatic rifles like the M-16 which operate from the closed bolt. I'd think that a case dwelling in the chamber is much more likely to draw heat from the barrel if that's a significant factor in the first place.
----------------------------------------

Automatic weapons typically fire from an open bolt because a FRESH cartridge resting in the chamber when firing ceases will heat up, and may "cook off."

griz
March 11, 2004, 06:58 AM
For what it's worth on weight, the plastic stuff weighs about 136 grains per cartridge while conventional rounds run around 170. Not as big a difference as I thought, but for 200 rounds that means less than 4 pounds versus 5 pounds for brass.

artherd
March 11, 2004, 08:24 AM
Kids- you're missing a critical point here.

OK, let's start off with a brass cartridge, with arbitrary numbers.

100 grains of powder driving a 250gr .338LM (bit hot for a .223 :) at 3,000fps.

80 watts of heat go into the barrel and muzzle blast, and 15 go into the just ejected case, and 5 go into the chamber indirectly through the case


Now a polymer cartridge.

82 grains of powder driving a 250gr .338LM at 3,000fps.

80 watts of heat go into the barrel and muzzle blast, and 2 go into the just ejected case. The case is such a good insulator, that effectively no energy goes into the chamber.


Can you guys see the benefit of this now?

ShaiVong
March 11, 2004, 09:44 AM
--------------------
I'm of the opinion that the brass case simply does a good job of COOLING THE IGNITION TEMPERATURE by sucking heat from the combustion in the chamber. I'd say that this factor has more to do with the brass heating than the theory that the brass is somehow acting as a cooling agent.
--------------------

But if you heat brass up to 120F, and a polymer case up to 120F, the polymer would have absorbed more energy, because it takes more energy to increase it 1 degree.

I think you have it backwards Vern.. Metals have a very low specific heat, which makes them good conductors, but that also means it requires very little energy to change their temperature.

The only limitation may be that the polymer case wouldnt stay in the chamber long enough to absorb energy as fast as brass would.. and thus brass may actually absorb more energy.

But considering the extreme heat caused by igniting a case, I would think more energy could be 'forced' into the polymer.

Third_Rail
March 11, 2004, 10:01 AM
artherd, your math is faulty. If 80 watts are created, plus 15, plus 5, we have 100 watts from 100 grains of powder (your numbers, not mine). Now, in situation B, we only have 80 watts plus 2. 100 versus 82, to drive the same bullet of the same weight at the same velocity? I don't buy that.

As a counter example, let's say both have 100 grains of powder, but neither case is full. So let's see, in situation A the numbers are the same, but in situation B, 98 watts are now in the barrel. I don't like that at all, and I'd imagine the rifle isn't happy either.

So I guess what I'm saying is that most people don't like to take into account the great effect of the added thermal mass of the brass, but it's still there.

But then again, I'm no expert.

EDIT: SV, think about this: if polymer is such a great insulator, does it not insulate itself? Layer upon layer of whatever composition they're using (I suspect a spectra/nylon polymer), each layer a wonderful insulator even from itself. Now, wouldn't the heat from the gasses be transferred to the barrel faster than it could be transferred to the polymer?

Vern Humphrey
March 11, 2004, 10:32 AM
Quote:
-----------------------------------------------
SV, think about this: if polymer is such a great insulator, does it not insulate itself?
-----------------------------------------------

Exactly. Which is why the polymer case will NOT carry as much heat as the brass case when ejected.

benEzra
March 11, 2004, 12:07 PM
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
March 11, 2004, 12:40 PM
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
March 11, 2004, 12:43 PM
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
March 11, 2004, 12:47 PM
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
March 11, 2004, 12:59 PM
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 (http://www.quarry.nildram.co.uk) and Discussion forum (http://forums.delphiforums.com/autogun/messages/)

MrAcheson
March 11, 2004, 02:28 PM
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
March 11, 2004, 02:44 PM
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
March 11, 2004, 02:56 PM
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
March 11, 2004, 06:12 PM
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
March 11, 2004, 07:03 PM
You're right, I started replying before you posted.

ShaiVong
March 11, 2004, 08:24 PM
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 :D

The_Antibubba
March 12, 2004, 05:16 AM
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, :o is this polymer really just a "plastic" or is it something else?

MrAcheson
March 12, 2004, 10:47 AM
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
March 12, 2004, 12:02 PM
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
March 12, 2004, 01:03 PM
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
March 12, 2004, 01:37 PM
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
March 12, 2004, 01:46 PM
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
March 12, 2004, 01:57 PM
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
March 12, 2004, 03:16 PM
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
March 12, 2004, 03:28 PM
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
March 12, 2004, 03:33 PM
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
March 12, 2004, 04:52 PM
"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
March 12, 2004, 04:57 PM
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
March 12, 2004, 05:59 PM
"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
March 12, 2004, 06:02 PM
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.

JWH
March 13, 2004, 12:05 AM
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
March 13, 2004, 12:20 AM
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
March 13, 2004, 04:36 AM
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
March 13, 2004, 05:07 AM
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
March 13, 2004, 01:45 PM
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
March 13, 2004, 07:00 PM
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
March 13, 2004, 09:11 PM
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
March 13, 2004, 09:55 PM
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
March 13, 2004, 10:17 PM
2002 International Infantry & Joint Services Small Arms Systems Section Symposium, power point presentation? (http://216.239.41.104/search?q=cache:-MTgJFg_BeQJ:www.dtic.mil/ndia/2002infantry/husseini.pdf+amtech+polymer&hl=en&ie=UTF-8)

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
March 13, 2004, 10:59 PM
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
March 14, 2004, 12:13 AM
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
March 14, 2004, 01:01 AM
Once again, I'd like to see statistics.

Vern Humphrey
March 14, 2004, 01:05 AM
We'd have to have statistics both ways, wouldn't we?

How do we get statistics on polymer cases?

Badger Arms
March 14, 2004, 03:59 AM
First we need money. Then we need to hire a lab. Know any philanthropists.

artherd
March 14, 2004, 05:34 AM
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
March 14, 2004, 05:39 AM

ShaiVong
March 15, 2004, 12:00 AM
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
March 15, 2004, 03:10 AM
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
March 15, 2004, 08:47 PM
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
March 29, 2004, 12:32 PM
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
March 29, 2004, 12:52 PM
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
March 29, 2004, 01:49 PM
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
April 7, 2004, 12:43 AM
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
April 7, 2004, 10:38 AM
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.

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