How Many Rounds Before Your AR Blows Up?

pdm

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The M-4 carbine, one of the primary rifles used by the United States military, appears destined for a change. After concerns surfaced about rifles overheating in a sustained firefight in 2008 in Afghanistan, the manufacturer and the United States Army are close to agreeing on a modification to the weapon’s barrel that makes the carbine more resistant to the stresses of extended firing.

The pair of videos shown below, taken at Colt Defense’s testing range, capture the thinking.

The first video shows an M-4 being subjected to an intensive sustained-firing test. The rifle used is the standard M-4 with a standard barrel. The weapon is secured on a bench and fed one full 30-round magazine after another without rest beyond the time it takes to replace empty magazines with full magazines.

Watch the video closely. After several magazines, the barrel smolders. Then it becomes red hot. After 1 minute and 20 seconds the barrel begins to droop between magazines — like a piece of warm licorice. Then comes the catastrophic ending, at 1 minute and 51 seconds and after the 535th round, when the barrel ruptures.

It is worth noting that the test simulates conditions that almost no soldier could face. In it, 18 magazines are fed through the rifle in less than two minutes. Soldiers and Marines armed with an M-4 or M-16 (the carbine’s longer-barreled parent) typically carry seven or so magazines.

Moreover, the M-4 carbine used in the test had been modified to fire fully automatically. As long as the shooter holds back the trigger, the test rifle keeps firing until the magazine is empty. Standard-issue M-4s fire only on semi-automatic or a three-round burst setting, not like this.

For these two reasons, it would be impossible for a soldier armed with a standard M-4 to fire as many rounds in such a short period of time — even with the ammunition, even with the desire. The rifle is undergoing a test similar to running an automobile engine at, say, 50,000 or more r.p.m.s.

Still, when set against the second video, the test has comparative value.

That video shows the same test with an M-4 equipped with a thicker, heavier barrel, which is used on a specialized carbine, known as the M-4A1. This variant is carried by some Special Operations users.

Naturally, the rifle still overheats. Heat is an unavoidable byproduct of the cartridges’ burning propellant. It cannot be avoided. But look at what happens with the M-4 outfitted with a heavier barrel.

The barrel gets hotter and hotter, and the heat spreads throughout the weapon. The shooter wears a heat-resistant glove even to pull the trigger. Soon the barrel smolders and glows, but it does not droop and does not rupture. At 2:22 the hand guard assembly catches fire. It burns for about two and a half minutes. But the rifle keeps firing, magazine after magazine, until it stops firing on automatic at 4 minutes and 47 seconds, after 911 rounds.

The reason for the stoppage is that the gas tube, which is located under the upper hand guard, has ruptured. The tube is essential. It diverts a portion of expanding gases associated with each discharged cartridge back toward the carbine’s bolt. This excess energy, aided by springs, is converted to the many steps required for automatic or semiautomatic fire.

With the gas tube ruptured, the shooter continues to fire the rifle several times manually. But at this point, more than 900 rounds after the shooting began, the rifle is a red-hot single shot weapon -– and no longer an infantry assault rifle that can perform as intended.

Even if the sort of extreme firing seen in these videos exceeds the rate of fire that can be achieved in combat, the takeaway is clear: increasing the thickness of an M-4’s barrel increases the rifle’s ability to function in sustained, intensive combat.

Colt Defense and the Army have been discussing making the change to a heavier barrel for several months and appear likely to begin requiring standard-issue rifles to have the barrel previously manufactured for the M-4A1.

If the change is made, the standard M-4 will retain its semiautomatic and burst modes of fire. It will not fire automatically.

Because both the lighter and heavier barrels are machined from identical sleeves of steel (the thicker barrel, in the simplest sense, spends less time on the lathe), the change can be made without increasing the cost per rifle.

The downside is that the heavier barrel would increase the weight of a standard M-4 by five ounces. The Army has all but decided the trade-off is worth it, and seems to be considering not whether it should require new carbines to be manufactured to this standard, but whether it should retrofit the hundreds of thousands of rifles already in the services’ possession.

“The bottom line is that we are going to do this,” Colonel Douglas Tamilio, who supervises small arms development for the Army, said of the change to new carbines. “We have to get all of the services to buy in, but it adds five ounces in weight and doubles your sustained rate of fire.”

He added, “I think it’s a no-brainer, and we’re going to see it in the near future.”

Videos at the link. I wept at the amount of ammo used. [wink]
 
Recently read an article about Hard Blue in the Varmint Hunter mag. They did a similar test on the same lower but two different uppers, one a regular and one that had the hard Blue process. The Hard Blue barrel lasted a whoel one thousand rounds while the regular stopped at 750. If you google Hard Blue you can find the article. The company that does the process is called Superior Metal I think.
 
I wish they tested weapon systems that see higher volumes of fire in battles. A M4 is not going to see as many rounds down range as a M249 or M240..
 
I wish they tested weapon systems that see higher volumes of fire in battles. A M4 is not going to see as many rounds down range as a M249 or M240..
At least per wikipedia that has been done and is presumably the source of the guidelines of 100 RPM sustained fire max:

http://en.wikipedia.org/wiki/M240_machine_gun

Type Rounds Fired MRBS MRBF
FN MAG 58 50,000 2,962 6,442
M60E2 === 50,000 846 1,669
M219 === 19,000 215 1,090
Minimum specified 850 2,675
Minimum desired 1,750 5,500

At any rate - the 500-700+ rounds isn't terribly interesting, nor is the barrel melting if the handguards are on fire - but that's just me [laugh]

Of course that's a clean gun in relatively sanitary conditions.
 
I was issued a new M4 (3 rd burst NOT full auto) on my way over to my latest tour of Iraq. I also picked up a Beta C-Mag, that first necessary 30 rds downrange always seem to go too fast. I had some real fun with that set up. Worst I encountered was blueing of the gas tube on the chamber end. Everything always performed as needed. Though, when I turned it in the civilian armorers were inspecting the weapons and gave the old "How the hell did this happen?!"

Here's some examples:

http://www.youtube.com/watch?v=dfAPkRiD3Fk

http://www.youtube.com/watch?v=wiiz1a4emrM
 
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like the article says M4 is the a 3-rd burst. M4A1 is the fully automatic variant. standard issue is m4, that's what i've been issued. i believe the M4A1 is for spec ops.
 
I remembering a day at Harvard about a decade ago, where I saw someone empty a fully loaded 100 round drum magazine through his M16 in one long burst.
Damn, it was something to see. Just thinking about it right now is putting an ear-to-ear grin on my face.
And they did to a second burst, about a half hour after the barrel stopped smoking.

Indefinitely sustained full auto fire, positively begs for a water jacket.
 
as far as i know M4's are full auto M16 a2 are 3 round burst my m4 at work is deff full auto.

M4s are 3 round burst, M4A1s are full auto. M16s also come in 3-round and auto. Id like to see a full auto M4. The MK-18 is a upper receiver group to be mated to a M4 or M4A1.
 
I remembering a day at Harvard about a decade ago, where I saw someone empty a fully loaded 100 round drum magazine through his M16 in one long burst.
Damn, it was something to see. Just thinking about it right now is putting an ear-to-ear grin on my face.
And they did to a second burst, about a half hour after the barrel stopped smoking.

Indefinitely sustained full auto fire, positively begs for a water jacket.

I have done a number of Beta dumps with my M16, melted plenty of handguards, "drooped" a few gas tubes never burst a barrel...Hell I actually have 3 beta mags and back when ammo was "cheap" would dump them one after another...I think most of us guys with 16's and beta's do it...I have no hard facts on it but most of us "MG guys" have heard the gas tube will ussually go between the 5th and 7th consecutive Beta dump, still never heard of the barrel failing, I've been to plenty of MG shoots where guns have red hot barrels and they still work fine...so if this test was popping barrels I suspect there may have been metalurgy issues...
 
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