In the enormous1 part one of the series, we reacted to a brain-dead article published in The Atlantic by a retired Major General, who has, since his retirement 20+ years ago, been a lobbyist for defense firms and TV talking head. (Before he got his stars he was an artillery officer). We may have more to say about our brain-dead GO in a subsequent post, but we think we raised some good points about his article. We weren’t the only ones. He also ticked off Nathaniel Fitch at The Firearm Blog, and we heard, also the guys at Loose Rounds (you know, the ones that fire M4s at 1000 yards and make the steel ring? Those guys?), and no doubt there are other places in the gunosphere flaying him. The point of today’s increment is not to make the rubble of the General’s small-arms expertise do a dead-cat-under-155-battery-closed-sheaf-fire-for-effect bounce, but to discuss the technical limits of a shoulder weapon in sustained automatic fire.
Because today is a travel day, this article was mostly-dictated for speed. Therefore, we fear we have some typos we haven’t found. Let us know in the comments.
Sustained Auto Fire and Heat
Many of the problems the M16A1 had in Vietnam, and even in adoption and acceptance prior to Vietnam, were caused by the heat of sustained autofire. It was particularly problematical after powder changes made a dramatic impact on the cyclic rate of the rifle. Indeed, Colt got a contract mod allowing weapons that had a much higher sustained rate than originally specified to be accepted.
Thermal waste is a huge problem for gun designers, and it’s been jamming automatic weapons since Maxim’s day. The heat is generated by the combustion of chemical powder in the chamber in barrel, but also by the metal-on-metal contact between bullet and barrel, which swages the impression of the rifling into the bullet and imparts a spin of hundreds-of-thousands of revolutions per minute to the bullet. The friction between bore and bullet is a significant contributor to barrel heating.
If you were in the service, you were made to memorize something about your rifle being a “shoulder-fired, magazine-fed, air-cooled, selective-fire…” weapon. The “air-cooled” seems like a historical artifact now; the last liquid-cooled small arms were the 1917 Browning machine guns, which were last used in World War II. All modern small arms of all nations are air-cooled. That means that the air around the barrel must carry the heat of the barrel away. Meanwhile, for each round, the barrel gets hotter, because firing’s ability to load up the temperature is greater than the cooling system’s ability to remove heat. (The original M16A1 had a patented passive design for convection-driven airflow, removing the heat from the holes at the top of the handguard and drawing new air in at the bottom. Designs since then have made efforts to maintain that cooling, with little success).
Because this post is long, and involved, we’re going to split it. Ahead, we describe the bad things that happen when barrels get hot; the results of M4 cyclic rate tests (including instrumented and well-documented tests to destruction), and Click “more” for the next three thousand or so words, a few pictures, and pointers to where you can find some of the math.
Bad Things Happen When Barrels Get Hot
The peak temperature area in the barrel is usually about three to seven inches forward of the chamber, depending on caliber (according to the references, on 5.56 mm rifles, it’s about four inches). This is where the thermal stress is at peak, and it also has to support all the rest of the barrel (and anything that may be attached to it, from a Surefire to an M9 bayonet), so when the gun is going to fail, it’s probably going to fail near here.
As more rounds are fired, more heat builds up, because it is being added at a higher rate than it can be radiated away. As the temperature rises, bad things happen:
- You have a risk of propellant cook-off. Weapons that fire from closed-bolt are especially prone to cook-off. At a critical temperature, the powder or primer will self-initiate. As the temperature rises, the amount of time a round has to sit in the chamber to heat-soak to the point that it self-initiates declines. At first it takes minutes, then seconds, then rounds can actually cook off before the automatic firing train fires them, and finally, they can cook off out of battery. Usually other damage disables the weapon by this point. This article at DTIC shows some of the tools the .mil has to model heat transfer, and compares predicted cook-off data to observed, unfortunately in a large-caliber small arm (30mm Mk44 vehicular cannon). They generated this equation (after Visnov) that shows :
Time to cook off (minutes) = 10.129 x 1025 x (cook-off Temp – degrees C) x 10-10.95
The cook-off temp is a constant for a given powder, and can be experimentally determined by heating the powder on a steel plate.
In the test, they did not maintain continuous fire but bursts of fire according to a firing table, then followed by letting a round sit in the chamber. Their cook-off times in live testing ranged from about 10 to about 30 minutes testing. Note that brass provides better protection from cook-off than aluminum cases, which in turn provide better protection than steel.
In another experiment, Hameed et. al. built a “Chamber simulator” and developed working chamber temperature-time curves for producing cook-offs in a 7.62mm brass case with Bullseye powder. They found that below 170ºC chamber temperature, cook-offs were unlikely, and that by about 240º, the cook-off time was down to seconds.
- It can cause failures to extract. This actually led to a change in the formula for Black Hills MK262 Mod1 ammo, as reported by Shooting Times:
[A]n improvement to temperature sensitivity came along in 2005. [Black Hills President Jeff] Hoffman said the last change came after Black Hills technicians noticed some failures to extract (FTX) in their test M4 and short-barreled rifles, and that it was the most difficult problem to solve.
“We initially thought the FTXs were possibly related to higher port pressures,” Hoffman said. “The M4’s port pressure is around 25,000 psi, much higher than the SPR due to the location of the gas port on the respective guns. We looked at brass, powder, everything.”
It finally came down to chamber temperature. The test specification called for the ammo to be baked at 125 degrees for two hours and not exceed pressure limits when then chambered and fired. When Black Hills engineers started firing test guns far beyond the specified rate of fire, the chamber temperatures got much hotter than 125 degrees. In an extended firefight, soldiers could heat up their rifles with a few mags, and then during a lull in fighting, a chambered round would sit in a 200- or even 300-degree environment. That significantly increased chamber pressures and induced failures to extract.
“After we figured it out, I was surprised that it hadn’t come up before,” Hoffman said. “We’ve gone from bolt rifles to eight-round Garand clips to closed-bolt, select-fire rifles. SF guys never had an issue because they are trained to fire two or three rounds per target and very rarely go full auto.”
It only took Black Hills 75,000 rounds to sort out the problem—a chunk of the 250,000 rounds Hoffman figures the company fired developing and lot-testing the load. Finally, the round was issued. Interestingly, the ammo always did meet specs, even the ammo that Black Hills engineers felt needed improvement—they just found a way to make it better. The Navy began changing test specifications based on what Black Hills learned—and shared—during development and testing. The improved round was a hit, no pun intended, with operators in-theatre, and usage went through the roof. Not only did the ammo perform well for its intended purpose—long-range shooting—but did equally well in short-barreled rifles like the M4 (14.5-inch barrel) and MK 18 (10.3-inch barrel), which leads to a discussion of lethality.
- It can cause the barrel itself to fail next time it is used. At a very high temperature, the barrel is heated until it loses its temper, which can cause an invisible (and undetectable by gaging) failure of accuracy. This was first noted with aerial machine guns in WWII, as we noted here before.
- If continued, it can cause the barrel to fail catastrophically whilst firing. Stripped of its heat treatment and heated to the metal’s plastic temperature, the barrel droops. At first, rounds extending through it will sort of “hold it up” but soon it will be unable to contain the pressure and will burst.
- If the barrel doesn’t fail first, heat can cause the gas tube to fail. Weakened by high temps, the tube lets go.
Any gun can cook off. The USN famously cooked off a 5″ on the destroyer USS Turner Joy in 1965 during a Vietnam War shore bombardment, killing three sailors and wounding three more.
Results of M4 Cyclic Rate Tests
Colt has, in fact, tested M4s at cyclic rate to destruction and has made these tests public. C.J. Chivers, a former Marine, has reported on these tests in a long and readable report for, of all things, the New York Times. That report was Part II of a previous report on M4 manufacturing there. We were unable to extract the Colt videos from the Times page, but it’s very much worth reading, anyway.
After the Colt tests, the Center for Naval Analyses did a report. We don’t have the report, but Kirk Ross at the US Naval Institute’s Proceedings Magazine did an excellent and thoroughly-documented synthesis of the then-known information, including the CNA report and the Colt tests, a DOD survey of weapons users, and SOPMOD program office documents. Ross’s article is an excellent short piece on these issues and we strongly recommend it.
A lot of what we know about the M4 under duress comes from mid-1990s research. In the 1990s, as the then-new M4A1 carbine began reaching special operations units that shot them a lot, they began blowing them up. In June, 1995, 10th SF Group had two cook-offs. In September, the 1st Battalion of the 1st SFG reported multiple problems, including cook-offs. In May, 1996, 7th Group blew one up in its then-home-station of Fort Bragg. In August, 1996, 3rd Group blew one up on an African JCET; one USSF was injured by gun shrapnel. 5th Group and the 1st Ranger Bat also blew up guns around this time, and that began to worry SOF soldiers and leaders — and the armament procurement guys. The Army resolved to test M4s to destruction to determine what was going on. The one thing they knew was that the destroyed guns had been fired a lot, primarily full-auto fire at cyclic rates, often “burning up” excess ammunition at the end of an exercise (wasteful, but the Army makes it very difficult to turn back in unused ammo, and the Air Force is snippy about transporting it).
In 1996, ARDEC’s Jeff Windham conducted tests-to-destruction to determine whether, as then rumored, M4 barrels were more prone to failure than the M16A2 barrel. These were early M4A1s with the M4 profile barrel (like the one we carried in Afghanistan), and the M16A2 controls in the test were modified to fire full-auto by subbing in M16A1 fire control parts. The guns were fixtured and fired full-auto. The intent was to fire one of each fully-instrumented weapon to failure. Initially, an M16A2 was destroyed:
The M16A2 was fired continuously using 30 rounds bursts. Shown in Table I are the rounds to failure, time to failure and maximum barrel temperature of the barrel. Muzzle flash increased and there was a distinct change in the sound of the weapons firing approximately 30 rounds before the barrel ruptured. There was also noticeable drooping (about 1 inch at the muzzle) of the barrel just prior to the barrel rupture. The barrel ruptured at 491 rounds with an approximately ½ inch hole in the top of the barrel about 8 inches in front of the chamber. The barrel was bent approximately 5 degrees and bulged in several locations along its length (see figures 4, 5, and 6). A plot of barrel temperature versus time at each thermocouple location is shown in figure 7.
Given the hypothesis that the M4 would die before the A2, Jeff fixtured the sacrificial M4A1 and set up 18 magazines, containing 540 rounds, and then fired them. But while the barrel was ruined, it didn’t actually burst:
The M4A1 Carbine was fired for 540 rounds. It was thought the M4A1 barrel would rupture well before this point, therefore only 540 rounds were loaded for firing. This weapon’s barrel was noticeably bent and bulged at the end of the test (see figure 8). A plot of barrel temperature versus time at each thermocouple location is shown in figure 9.
Oops. Back to the testing bench, with another M4A1 selected as a sacrifice to the gods of knowledge.
A second M4A1 Carbine was fixtured for testing and fired until barrel rupture. Muzzle flash increased and there was a distinct change in the sound of the weapons firing approximately 30 rounds before the barrel ruptured. There was also noticeable drooping (about 3/4 inch at the muzzle) of the barrel just prior to the barrel rupture. The barrel was ruptured at the 12 o’clock position approximately 4 inches in front of the chamber. The rupture was approximately 1V4 inches long and 5/8 inches wide. The barrel around the rupture was bulged out about 30 percent larger than its normal diameter. The barrel was bent at the hole approximately 3 degrees (see figures 10 and 11). A plot of barrel temperature versus time at each thermocouple location is shown in figure 12. There was an approximately 30-second delay in firing of this sequence which can be seen in the temperature plots. This delay allowed additional cooling of the weapon and may have increased the number of rounds to rupture by 30 to 60 rounds.
Here is the Table 1 from the report. The other figures and tables referenced in the quotes are in the report, which is linked in the Sources below, although the photo reproduction is of very low quality.
SOCOM sent a safety message as far back as 1996, presumably based on Windham’s research (although we didn’t notice if they said that) about cook-offs with sustained fire. It is reproduced in this archived ARFCOM thread. We recall receiving this message with a red-bordered safety cover sheet. The thread poster has good advice. Here are a couple of lines from that message:
Sustained firing of the M16 series rifles or M4 series carbines will rapidly raise the temperature of the barrel to a critical point.
Firing 140 rounds, rapidly and continuously, will raise the temperature of the barrel to the cook-off point. At this temperature, any live round remaining in the chamber for any reason may cook-off (detonate) in as short a period as 10 seconds.
Sustained rate of fire for the M16 series rifles and M4 series carbines is 12-15 rounds per minute. This is the actual rate of fire that a weapon can continue to be fired for an indefinite length of time without serious overheating.
The sustained rate of fire should never be exceeded except under circumstances of extreme urgency. (Note: a hot weapon takes approximately 30 minutes to cool to ambient temperature conditions).
Cook-offs out of battery result from a round which cooks off when the bolt is not locked or a round which cooks off as the user is trying to clear the weapon.
Burst barrels result when the weapons are fired under very extreme firing schedules and the barrel temperature exceeds 1360 degrees Fahrenheit. When the barrel reaches these extreme temperatures, the barrel steel weakens to the point that the high pressure gases burst through the side of the barrel approximately 4 inches in front of the chamber. This condition can result in serious injury.
That is, of course, exactly the failure mode in the first M4 video at Chivers’s report. And this is from a message from 1996, so SOCOM’s weapons experts knew it almost 20 years ago, and more than 10 years before Wanat.
600-700 degrees F is where cook-offs begin, and that’s reached in as few as 140 rounds on rapid semi-auto fire.
Here’s a table with some key temperatures for you:
|Temp F||Temp C||Rounds||Comment|
|230||110||30||semi-auto M855 in M4|
|278||137||30||full-auto M855 in M4|
|600||316||140||semi-auto; threshold of cook-off|
|700||371||?||frequent cookoffs, barrel weakened|
|1360||737||~500||semi or full, catastrophic failure|
|© Weaponsman,com 2015|
How to Deal With Heat Limits
The Training Answer: First, every GI should see those Colt test videos and know what his gun can, and can’t, do. While the Black Hills guys were correct in noting that SF/SOF guys usually manually fire single shots or short bursts, even most of them don’t know what happens when a gun goes cyclic for minutes at a time. A good video explaining “why you can’t do that” would be a strong addition to training, not only for combat forces, but for support elements who may find themselves in combat and feel the urge to dump mags at cyclic rate.
The Morale Answer: Every GI should see the same done to AKs as well. There is a myth perpetuated by pig-ignorant people (like General Scales) that the AK series possesses magical properties and that the American weapons are crap. In fact, nobody I know of at the sharp end is at all eager to change, perhaps because the laws of physics and the properties of materials apply just as firmly to a gun originally created by a Communist in Izhevsk as they do to a concept crafted by capitalists in California. If you’ve ever fired an AK to destruction, you know that it grows too hot to hold, then the wooden furniture goes on fire, then, if you persist on firing it full-auto, it also goes kablooey. Not because there’s anything wrong with this rifle, but the laws and equations work the same for engineers worldwide.
The Systems Answer: As you can see from the Colt videos, if you clicked on over to Chivers’s article, thickening the barrel nearly doubled the rounds to catastrophic failure on cyclic. An open/closed bolt cycle might have practical benefits. They wouldn’t show up in sustained heavy firing like the destruction tests, but they might show up in how a weapon recoups from high temps, and open-bolt autofire would eliminate cook-offs, at least. But any such approach needs thorough testing.
The Wrong Answer: Replacing the M4 with something like the SCAR or the HK416, something that is, at best, barely better, that is much more maintenance intensive, and that, contra Scales’s assertion that his undisclosed client’s weapon is “the same price,” is twice (SCAR) or three times (416) the money. (The 416 mags are the best part of the system, though).
It would be interesting to duplicate Jeff Windham’s M4A1 destruction tests with AKs and with other competitors, like the 416. Scales says a piston system like those (never mind that each one is a very different design) would not fail under the conditions seen at Wanat. We’ve seen from the information here, that the failure of firearms under high rates of fire is driven by the physical problems of waste heat and metallurgy. Our prediction is the laws of physics apply in Russia and Germany as well.
Did Weapons Cause Deaths at Wanat?
We’ve talked about how the weapons fail, when they fail, today. But in the previous post, we were looking at this in the context of a very important question: did weapons deficiencies cause deaths at Wanat? We reached our conclusions. In The Atlantic, Major General Scales, the undocumented lobbyist and long-retired talking head, reached the opposite conclusion, and asserted that the nine fatalities that day resulted from, specifically, M4 failures. We are not sure whether his problem is lack of familiarity with the material we’ve presented here, or whether it’s an integrity issue, but we think we’ve rather conclusively made the point that any honest answer comes back, “No.”
But it’s worth noting what the other investigations decided.
- The historical investigation, both the Cubbison and the final, come up, “no.”
- The RAND report does not fault the weapons. It does suggest some theoretical future weapons developments, such as miniguns or thermobaric weapons, and points out the dead-space problem without making a specific suggestion of how to address it.
- The Army 15-6 investigation, came up “no,” and said so explicitly.
- The DOD Inspector General investigation, that was extremely critical of the leadership of the company, battalion and brigade, did not mention weapons as a factor.
And so we’re not really in bad company, even though were on the other side of a Major General on this.
1. A good web article is about 300 words. A good newspaper column is about 700 words. Because we have faith in our readers’ ability to follow pieces of greater length and complexity, we frequently go to 1000 or even 2000 words (although our mean comes in around 600). That article was 3,129 words. And well illustrated, too.
Chivers, CJ. The Making of the Military’s Standard Arms, Part II. New York Times (online): 12 Jan 2010. Retrieved from: http://atwar.blogs.nytimes.com/2010/01/12/m4-and-m4a1-guns/?_r=1
Department of Defense. MIL-STD-3029: Department of Defense Test Method Standard: Hot Gun Cook-Off Hazards Assessment, Test and Analysis. Washington, DC: DOD, 23 July 2009. Retrieved from: http://everyspec.com/MIL-STD/MIL-STD-3000-9999/download.php?spec=MIL-STD-3029.022917.PDF
Guthrie, J. Reviewing Black Hills’ MK 262 Mod 1 Ammo. Shooting Times: 21 Mar 2012. Retrieved from: http://www.shootingtimes.com/ammo/special-forces-to-civilians-black-hills-mk-262-mod-1-review/
Hameed, Amer, Azavedo, Mathew, and Pitcher, Philip. Experimental investigation of a cook-off temperature in a hot barrel. Defence Technology.Volume 10, Issue 2, June 2014 (28th International Symposium on Ballistics), Pages 86–91. Retrieved from: http://www.sciencedirect.com/science/article/pii/S2214914714000385
Ross, Kirk. What Really Happened at Wanat. Proceedings Magazine, July 2010. Vol. 136/7/1.289. Retrieved from: http://www.usni.org/magazines/proceedings/2010-07/what-really-happened-wanat
Smith, Herschel. The Captain’s Journal. Multiple posts on Wanat linked to his Battle-of-Wanat category. Basically, Hersh has beaten all this ground years before (and we’ve even cited his reports here, before). Retrieved from: http://www.captainsjournal.com/category/battle-of-wanat/
Windham, Jeff. Fire To Destruction Test of 5.56mm M4A1 Carbine and M16A2 Rifle Barrels. Rock Island, IL: Engineering Support Directorate, Armament Research, Development And Engineering Center. September, 1996. Retrieved from: http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA317929 (Abstract: http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA317929).
Witherell, Mark, & Pflegl, George. Prediction of Propellant and Explosive Cook-off for the 30-mm HEI-T And Raufoss MPLD-T Round Chambered in a Hot Mk44 Barrel (Advanced Amphibious Assault Vehicle – AAAV). Watervliet, NY: Army Research Laboratory/Benet Labs, March 2001. Retrieved from: http://www.dtic.mil/dtic/tr/fulltext/u2/a388280.pdf
Wanat General References from Part 1
Combined Joint Task Force-101, “Army Regulation 15-6 Investigation into Battle of Wanat (Redacted, Unclassified Version)” (Bagram Airbase, Afghanistan: 21 October 2008.
Cubbison, Douglas R. Occasional Paper: the Wanat Operation (first draft). US Army Combat Studies Institute, 2009. Retrieved from: http://www.washingtonpost.com/wp-srv/special/world/battle-of-wanat/correspondence/documents/2-Army-Historians-Report.pdf
DOD Office of the Inspector General. Oversight Review of the Reinvestigation into the Combat Action at Wanat Village, Afghanistan. Arlington, VA: 22 July 2010. http://www.dodig.mil/ai/reports/roi-wanat508.pdf
Staff of the US Army Combat Studies Institute. Wanat: Combat Action in Afghanistan, 2008. Leavenworth, KS: US Army Combat Studies Institute, 2010. Retrieved from: http://usacac.army.mil/cac2/cgsc/carl/download/csipubs/Wanat.pdf found at: http://usacac.army.mil/organizations/lde/csi/pubs.
Steeb et. al. Perspectives on the Battle of Wanat: Challenges Facing small Unit Operations in Afghanistan. Santa Monica, CA: RAND Corporation, 2011. Retrieved from: http://www.rand.org/content/dam/rand/pubs/occasional_papers/2011/RAND_OP329z1.pdf