M4 Endurance Testing

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Bartholomew Roberts

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I had the opportunity today to look at a few 5-gallon bucket fulls of Colt M4 uppers (14.5") being used for endurance testing. The uppers are being fired full-auto for 210 rounds with the only break being a quick mag change every 30rds. After 210 rounds, the uppers are removed and dropped into the 5-gallon bucket to cool down to ambient temperature.

The uppers receive a detail cleaning every 4,500 rounds.

Two of the barrels (out of maybe 20? uppers) had ruptured at the portion midway underneath the handguards. One before its first cleaning and one at around 6,000 rounds (just in case you were curious how long an M4 barrel can survive an extreme firing schedule).

The people doing the testing indicated such ruptures were not unusual and at least one of the test uppers had been replaced with a thick HBAR style barrel (no M203 cuts).

Since few of us get the resources to test uppers in such a manner, I thought folks here might appreciate an idea of what an M4 barrel is capable of handling.
 
Thank you for posting this. :)

Perhaps it will help clear up a few misconceptions... ;)
 
Yeah, I would like to hear more about how the barrels ruptured? Was this a kaboom-type rupture with an obstruction? Or was this due to a stress fracture from weakening of the metal at high temps? Why were the barrels replaced with an HBAR? I always thought that HBARs were turned down under the handguards also.

I would also be most interested in hearing about FTF's or FTE's. Sure since they were AR's they must have failed every other round after 3 mags, right? ;)
 
Oh No

The M16 Haters Club will now have more ammunition to bore us with.
The M4 is a bust. They can only be fired full auto for 4500 rounds before the barrel MIGHT let go.
:banghead:
 
I'm wondering if there was any accuracy testing done along the way.

I'd specifically wondered if the M4 barrel,w/all its different cuts/profiles,had any propensity to REALLY change POI vs. the same rifle w/a HBAR profile?
 
I always thought that HBARs were turned down under the handguards also.
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HBARS are narrowed a little right after the barrel nut, then taper very slightly all the way down to the muzzle.

M4 and A2 "government profile" are narrowed down a lot after the barrel nut, all the way under the handguards up to the gas block. Then they are thick again from the gas block to the muzzle.

A1s/SP1s are narrowed from the barrel nut all the way to the muzzle.

Bull barrels are just thick for the entire length.


The people doing the testing indicated such ruptures were not unusual and at least one of the test uppers had been replaced with a thick HBAR style barrel (no M203 cuts).
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Doesn't the military issue some M4s with HBARS and full auto capability (instead of three round burst) for special forces?
 
I had a chance to speak with an Army officer in charge of purchasing weapons for the Army. He said that they are having issues with the life expectancy of the weapons. He would not go into detail and I didn't puch too much.
However, he closed the discussion by saying that the Marine Corp never passed them and never bought any. I think that is true, I don't recall any Marines with them.

Not to beat up the M4. However, the shorter stroke seems to be hard on the system. I really doubt any civilian is going to wear one out but the extremes should be known.
 
The better question is why they were doing endurance testing when the M4 has been in the process of being purchased for a while now. Usually you do endurance testing prior to plunking down a couple hundred million for a new weapon systems. I'd like to know more details on the test, where at, what unit, what level, and for what purpose.
 
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To answer a few questions:

Can you clarify 'rupture'?
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As in blowed up! :) The barrel is blown outwards with a larger bullet sized hole in the middle and "ribs" of steel around the larger hole that look like bars on a cage.

And in addition to detail cleaning, are the bolts being inspected and gas rings being replaced?
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We didn't discuss this, so I couldn't say.

Yeah, I would like to hear more about how the barrels ruptured? Was this a kaboom-type rupture with an obstruction? Or was this due to a stress fracture from weakening of the metal at high temps?
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I wasn't present for either rupture and no investigation was done as to why they ruptured. Mostly it appeared to be a heat issue where the barrel had become hot enough to droop. No obstructions were at issue (unless you count the drooping barrel as the obstruction). The ruptures looked almost as if the bullet exited through the side of the semi-molten barrel along with a lot of gas.

Why were the barrels replaced with an HBAR?
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All of the barrels but one were 14.5" Colt chrome-lined military M4 barrels. One of the barrels in the bucket undergoing testing was a very heavy (heavier than a Bushy HBAR in profile) 14.5" barrel. Don't know why it was there amongst the rest; but it was. Haven't ever seen a similar barrel offered commercially and it definitely wasn't the heavy M4 profile offered for the M4A1.

I would also be most interested in hearing about FTF's or FTE's. Sure since they were AR's they must have failed every other round after 3 mags, right?
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Not much problems with FTF or FTEs; but apparently after about 160,000 rounds the fire control pins egg out the lower receiver and need to be replaced with larger diameter pins or they will fall out of the gun when it is flipped on its side.

I'm wondering if there was any accuracy testing done along the way.
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No

I'd like to know more details on the test, where at, what unit, what level, and for what purpose.
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Sorry; but I'd hate to get my sources in trouble after they were kind enough to share the info with me. Suffice it to say that the item being tested wasn't the M4 and that the info I was able to pick up was just incidental information not covered by the NDA. It was also not a governmental agency doing the testing.
 
I've seen similar boxes of burst M4 barrels also in TX.

You can't expect an assault rifle barrel to work as a light machinegun barrel. It's as simple as that.

The better question is why they were doing endurance testing when the M4 has been being purchased for a while now.
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Makes you wonder if the M4's were the subject of the testing, doesn't it?

-z
 
My 6920 will never get that abuse. I like to think it could handle it if it did however.
 
I recently read a report about some M4 barrel testing.
It said the M4 barrel would burst within a narrow range of number of rounds fired in sustained full-auto fire.

According to the report, the sound of the rifle noticeably changed within about 20 to 30 rounds of the burst.

This test was conducted by firing full-auto as fast as the weapon could be loaded, with no pause, until the barrels literally began to glow and slump from the heat.
At this point, the sound of the weapon would change, and within that magazine load, the barrel would burst.

According to the report, the purpose was to investigate whether the M4 carbine was less durable than the standard M16 rifle.

According to the report, the rifles burst BEFORE the M4's did, but not by much.
According to the report, the M16 barrel would burst about one to two magazines before the M4 would.

I think the keys here are:
NO weapon can resist sustained, abusive full-auto fire without barrel damage.

The M4 will last "about" as long as the M16 rifle in sustained fire.

It's very unlikely any operator could fire a weapon that much due to heat buildup.

I suspect the purpose of this testing is to settle rumors that the M4 can't withstand a reasonable amount of full-auto firing.
 
The barrels were heated up from extreme rapid firing, then dunked into a bucket of water to cool, right?

I'm no metalurgical expert, but wouldn't that kind of rapid transition from very hot to very cold mess up the temper or heat-treatment of the steel? Is it possible that the barrels burst because they were compromised by this method of cooling, and not by any particular defect of the design?
 
In the report I read, there was no quenching in water.

An upper, stripped of the handguard, was put into a test fixture then fired until it failed.
 
The M16 Haters Club will now have more ammunition to bore us with.
The M4 is a bust. They can only be fired full auto for 4500 rounds before the barrel MIGHT let go.
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I have never heard of an AK doing that (flame suit on)
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Well, at least it didn't take long. ;)

I somehow feel completely comfortable knowing that my M4-profile barrel will go "only" 4,500 rounds of full-auto without cleaning before it might fail. I understand the durability argument, I really do. Its nice to know that an item is capable of doing something that you would never ask it to do, and if there was otherwise no difference between the two items in question it might sway judgement. However...

Mike ;)
 
Just for way of a benchmark, even actual machineguns-like the M249 SAW and the M240B-aren't supposed to be fired that much in sustained fire without changing the barrel. And for exactly that reason. Doctrine on the MGs says to change the barrel after 1000 rounds of sustained fire. Barrel #1 cools while you're firing through barrel #2.

Wonder what similar torture tests on those weapons would reveal? Of course, you could burn through the ammo quicker, since you could just link it all together and hold the trigger down.

Fun trivia fact for the day: the ready ammo box for the coaxial M240C MG (the coax is mounted coaxial to the 120mm main gun) holds 10,000 rounds. Imagine having to link and load all that (7.62 comes in 100 round belts).
 
I have never heard of an AK doing that (flame suit on)
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Which only means that you don't know anyone with a full-auto AK that was willing to test the weapon to destruction. <yawn>
 
Makes you wonder if the M4's were the subject of the testing, doesn't it?
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Well color me ignorant, but if they weren’t testing the M4s (which I agree- it’s a little far along in the procurement process to start doing that sort of test-to-destruction stuff) then what could they be testing?

Ammo is the only thing I can thing of off hand. But what kind of ammo test is determinate upon the barrel life?

And did I word that question correctly?
 
Not much problems with FTF or FTEs; but apparently after about 160,000 rounds the fire control pins egg out the lower receiver and need to be replaced with larger diameter pins or they will fall out of the gun when it is flipped on its side.
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Never seen that, the FCG pins are held in place by the related springs, not the friction of the lower. Enlarged holes lead to improper geometry between the trigger, hammer, and disco. Do the larger pins require different FCG parts?

So testing reveals that I shouldn't treat an assault rifle carbine as a medium machine gun by operating it in excess of its published sustained rate of fire. Got it. :)
 
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