M4 Malfunction: Carbon Seized Bolt

Aug 31st, 2009 | By | Category: Tips and Tricks

A couple days ago I got a call to help out one of our AggDefSol friends. He was having trouble with his duty M4 carbine. He could not get it disassembled to clean it. Specifically, the bolt carrier was stuck in the upper receiver.


 I spoke to him on the phone asked a bunch of questions and ran through the typical troubleshooting steps.

  1. Brute strength. (that didn’t work)
  2. Break-Free and more brute strength. (that didn’t work either)
  3. Sharply striking the butt against the ground while holding the charging handle. (Caution: this is not normally recommended unless all else fails. You could break your stock or worse bend your buffer tube. Also, it is important to strike the stock in line with the buffer assembly to prevent putting excessive torque on the buffer tube.)

Our friend is quite adept and was relieved that he had already done all the quick tricks I could have him try over the phone so, I had him bring the carbine over and I quickly ascertained:

partial-bolt-turnWhile the bolt carrier would not come out of the upper receiver, the bolt carrier did not appear to be binding or stuck. You can determine this by wiggling the back of the bolt carrier and checking for movement due to the “slop” in the AR systems’ tolerance.

We could get the bolt carrier out of the upper about 1/8 of an inch using brute force. This happens to be the length of travel of the bolt in the bolt carrier before the carrier key forces the bolt to rotate. So, I assumed the bolt and bolt carrier were not seized together.

Using the bolt-over clearing drill (see number 3 above) I was able to get the bolt carrier out 1/4 of an inch and could see the bolt beginning to rotate by observing the chamber. At this point I was worried that if I was wrong and the issue was not just the bolt binding in the chamber then I would get the bolt carrier stuck inside the buffer tube thus creating a whole different problem.

Finally, he put on a lot of break free on the bolt and bolt carrier. The whole thing looked like a muddy mess. This was not nice clean lubrication , but thick, black carbon soaked muck. What a joy to work on.

 DSC06970The rubber mallet is your friend

 I have an armorer buddy who is fond of telling people, “You may want to leave the room before I hit your gun with this hammer.” I find a rubber mallet makes people a little less squeamish

 Finally, we blocked the upper receiver and used a large slotted screwdriver against the bolt carrier; with just two taps from my rubber mallet the bolt carrier came sliding out of the upper receiver. Note: take care to have the tip firmly against the carrier to prevent chipping or slipping.

Why did it happen?

 It is important to explain the condition of the carbine to ascertain why the bolt was stuck. As I said, the break free was a mucky mess. He was disassembling the carbine to clean it, but it was last fired two weeks earlier. (OK, I will address that in a bit.) He fired approximately 1000 rounds during the training day; in addition, the training was tactical in nature, the kind that gets dirt in the gun. When he was done on the range he put the carbine, bolt locked forward, into his gear box, cleaned up the range and stowed gear. Two weeks later he went to clean the carbine. The AggDefSol staff discussed this incident and we believe the cause of this seizure was the excess of carbon, sand and brass flakes which built up over the course of the thousand round day. This amount of ammunition caused the metal in the chamber to expand, yes, our guns do that and continue to run. The carbon baked on basically turning into black concrete; I confirmed this while examining the bolt and firing pin. After the training day was over the carbine cooled and the metal contracted causing the sand, carbon and brass flakes to be compressed between the bolt and the chamber, making normal manual extraction impossible.

How to prevent

  1. Fully clean your carbine immediately after training; especially, if it’s a duty weapon, your life depends on it.
  2. Quick field clean. Often we will do a quick internal inspection after class. This requires a quick wipe down of the bolt face to check for damage and a swipe of the chamber to look for damage or metal wear. This check and “cleaning” can keep your carbine running a few hundred more rounds.
  3. Don’t store a hot gun on a closed bolt. We can also think safety here. When clearing and cleaning the range leave you carbine with the bolt locked to the rear. It is a visual safety check and it allows the gun to cool without the binding effect.

Any thoughts on how to do it better? Leave a comment and let us know.

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Article by Lee

Website: http://aggdefsol.com

Lee retired from the U.S Army in 2008 as a First Sergeant where he saw combat action during both Desert Storm and Iraqi Freedom. He led a physical security detachment in Central America; subsequently, ran a law and order detachment in Europe. Post 9/11 he was detailed with installation physical security which included both military and DA police training and operations, local and state law enforcement liaison.

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  1. have RRA lower / colt 16″ upper,had round get stuck above bolt carrier,bolt was not closed all the way,slammed stock on ground,pulling charge handle same time,worked.striped ar ck everything with lite did not see any damage changes mags fired a few more rds no problems, what caused this?bad mag?gun is properly lube.[ worked as armmorin past] never seen this before.had a oly car yrs ago rounds would get stuck in chamber pull trigger hear click,could not open bolt placed stock on ground pointing muzzle away from face stricking charge handle with boot heel [worked] rd would would be bent a little but would fire if placed in chamber by hand,found out it was gas tube,changed it no more problems.would like to know what caused the above problem?

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  2. Try Slip 2000 EWL. I used to have issues with fouling, but since I switched to EWL I haven’t had any issues. It make CLP seem like WD40. Every 500 rnds just add a few drops through the ejection port and you are good to go. Keep your bolt lubed and you will not have the issues. You wouldn’t run your car with out oil, why run your carbine that way. An average weekend of training will run @2,000 rnds and no issues.

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