This article was originally posted on Thefirearmblog.com
In the comments section of my recent Brief Thoughts article regarding caseless ammunition, there was a discussion about whether the cookoff issues of caseless would also be problem for LSAT-style polymer cased telescoped ammunition. Based on conversations I have had with subject matter experts regarding polymer cased ammunition in general, I noted that a lower cookoff threshold is one of the challenges I would expect CT ammunition developers to face. However, after some back-and-forth in the comments, I decided to contact LSAT/CTSAS program officer Kori Phillips regarding this issue (as it was not something I covered in my three-part interview with her), and she kindly agreed to allow her comments on the matter to be published here on TFB. They are below:
Nathaniel: One of my commenters recently mentioned cookoff testing that had been conducted for the LSAT program, specifically this slide from your 2011 NDIA presentation.
I am interested in learning more about this cookoff testing and its results. Speaking with various sources about polymer cased ammunition, I have learned that typically the cookoff limits are lower due to the reduced heat evacuated from the system since they lack the brass case (which acts as a heat sink). However, it also seems that the polymer case itself acts as an insulator, which should aid the cookoff limit.
Can you tell me more about the fixture and procedure that was used for this test? Was the chamber a test chamber, was it a standalone fixture as with the early caseless testing you did, or was it a barrel? Was the chamber heated continuously, or heated to 300 degrees and then allowed to cool?
I was curious if you thought that CT had a major advantage in this respect (cookoff) vs. the polymer composite cased configuration, and if so, why?
Kori: We have found that through a combination of the polymer cartridge case and, more importantly, the chamber being separate from the barrel, that the chambers of the CT weapons never get much beyond 200 degrees F, which is not enough to auto ignite the ammunition, no matter how many rounds you fire. The notion that heat gets “evacuated” with the brass case is overstated, because what happens with a polymer case is while the heat of combustion is the same, it is actually trapped in the case because the case acts as an insulator. Polymer cases are cool enough to pick up as soon as they get ejected, but they are smoking hot on the inside (and don’t touch the primer cup, I know that from experience).
We did not use a test fixture for these tests – they were actual weapons. As it states on slide 10, we fired 300 rounds with no stoppages through the LSAT 5.56mm LMG, at a rate of 76 rounds/min. We then chambered the 301st round and let it sit for 30 min. The only tricky part is getting that last round into the chamber without automatically firing it (I think they used a deadened primer so it wouldn’t fire, but I don’t remember for certain).
If it is [polymer cased conventional ammunition] being fired from a legacy weapon (i.e. the chamber and the barrel are one piece) and/or the ammunition has a metal base, this does increase the probability of cookoff.
Nathaniel: It sounds like maybe the chamber absorbs heat from the barrel as well? Also, maybe the metal base acts as a conductor and can ignite the propellant? So the ideal configuration for a backwards compatible polymer cartridge would be to have as much polymer as possible, even down near the base – which presents it’s own problems, it seems.
Could you elaborate a bit more?
Kori: You are 100% correct – in a legacy weapon, the chamber and the barrel are basically the same temperature (and get to around 600 degrees F, if I remember right). Whatever metal is on the case will quickly get to that temperature too, eventually lighting the primer and the propellant if they are in contact.
Nathaniel: Do you know why the chamber doesn’t get as hot on the separated designs? Is it because most of the heat is deposited in the middle of the barrel somewhere? I seem to recall something like that from destructive tests of M4s that I read.
Kori: The barrel gets really hot on its own – there’s plenty of argument for why (it’s either burning propellant or the friction of the bullet getting engraved by the rifling, or most likely, some combination). Since the CT chamber is “insulated” from the burning propellant by the polymer case and physically separated from the barrel, it stays cool. You can actually touch it with your bare hands after firing.
Nathaniel: Thanks a lot for answering my questions!
The short version is that current CT weapons actually have a higher cookoff threshold than conventional brass-cased weapons, for the following reasons:
- The majority of the heat dump occurs midway down the barrel, and the separate chamber of CT weapons isolates the chamber from that.
- The polymer case totally envelopes the cartridge, insulating it from heat and giving it no conduit to ignite the primer or propellant.
One source of confusion comes from testing of other polymer cased ammunition designs, specifically ones that fit and fire in conventional guns. The majority of these rounds use a high wall metallic base, similar to a shotgun shell, which provides a direct conduit for heat to the primer and propellant. Since the propellant is just as exposed to the heat of firing as it would be normally, but less heat is evacuated when the case is ejected, the cookoff threshold is correspondingly reduced. Since its configuration is different, CT does not have this problem. This raises the question of whether a “full polymer” composite case design with only a metallic rim could be a viable design for retrofitting existing weapons, or whether that would introduce its own problems.
(Ostariusalpha, maybe I owe you a beer?)