Getting ready to trouble shoot a Colt M4

[packinheavy]

We had a M4 start acting funny last weekend on the range at Ft. Lost in the Woods when we were doing qualification. We were doing predeployment weapons training for some of our people.

It would fire three rounds and then fail to fire the 4th. The shooter would then have to manually charge the weapon ejecting a live round from the chamber. She then could shoot three more rounds but would then have to manually charge the rifle again.

I didn't get a chance to crack the gun open on the range to see if the hammer was following or being held by the disconnector. It sound like something is screwy with the burst cam though.

The rifle is a standard issue Colt M4 with I am guessing less than 2000 rounds thru it.

I will post what I find and if any of you guys have had similar happen please share your experience.

[Failure2Stop]

More information might help-
define "fail to fire".

Did the hammer drop or was the shooter unable to drop the hammer?
Was the bolt going fully into battery?
What did the ejected round look like, specifically the primer? (assuming that the round was live and not a fired case)
Was a function check done on the weapon?

I have seen similar instances with a broken and worn hammer springs, in these instances the hammer would fall but not fire the primer reliably.

It doesn't sound like a burst-cam issue, but would need to see the FCG and cam to really make a call on that.

[packinheavy]

More information might help-
define "fail to fire".

Did the hammer drop or was the shooter unable to drop the hammer?
Was the bolt going fully into battery?
What did the ejected round look like, specifically the primer? (assuming that the round was live and not a fired case)
Was a function check done on the weapon?

I have seen similar instances with a broken and worn hammer springs, in these instances the hammer would fall but not fire the primer reliably.

It doesn't sound like a burst-cam issue, but would need to see the FCG and cam to really make a call on that.

Unfortunately I was not her instructor on the line so I didn't get to see it in action. Most of what I know is second hand. I was down a few shooters from her. But since I am the full time guy I will be working on it.

I don't know if the hammer was dropping or not. Three rounds would fire on semi regular as clockwork and then the fourth would not. She would manually charge the weapon and she could fire three more. This happened every time. I am leaning towards the hammer not dropping, but I am not 100% sure on that. I will be giving the shooter and her instructor a call later today to get more information.

The bolt was going fully into battery. She was performing SPORTS as trained.

I have no idea on the condition of the rounds when ejected. They were reloaded in the weapon and fired to complete the course with her instructor loading them into another magazine. This is another thing I will ask her instructor about.

No idea if a function check was performed on the range before or after the problem. The weapon passes one now though.

I have the lower sitting next to me as I type this and I am stumped. I can't find anything wrong with it. It is a little over lubed, but that is it.

These M4s are practically new(3 years old). We are an Air National Guard Security Forces unit so sadly the guns don't get shot more than twice a year unless they are used in predeployment training as they are now.

I will have to get it out the range this week to fire it and see if it is working properly. It passes a function check with no problems so it should. We will see.

The good news is that these weapons will not be going on the deployment. They will be issued different ones on their arrival that they will rezero.

Thanks for the help so far.

[packinheavy]

A little more information.

The burst cam is advancing properly.

The disconnectors are moving freely as is the auto sear.

The hammer spring does not feel weak.

In short, everything seems to be fine. :confused:

[Failure2Stop]

When you say it would fire three rounds, then fail to fire, how many rounds total were fired? I am curious simply because I wonder if it really is a set pattern or simply an anomaly appearing to be a pattern.

Have you checked firing pin protrusion?
Head-space?
Action spring length?
What buffer weight?

I would swap the hammer spring with a new one just to be sure.
All I can recommend at this point is that you test-fire the weapon since the information you received is incomplete/lacking detail and all else seems to be in order. I would test-fire about 30 rounds slow fire, 30 rounds rapid fire, and 30 rounds burst.

I am interested to see what you discover during the test-fire.

[Abraxas]

These M4s are practically new(3 years old).

The good news is that these weapons will not be going on the deployment. They will be issued different ones on their arrival that they will rezero.

Thanks for the help so far.

Must be nice to have that kind of funding. My first trip to the sand box I had duct tape keeping the pins that held my SAW together, and only one of my barrels had rifleing the other had transitioned into a smooth bore :rolleyes:

[packinheavy]

Must be nice to have that kind of funding. My first trip to the sand box I had duct tape keeping the pins that held my SAW together, and only one of my barrels had rifleing the other had transitioned into a smooth bore :rolleyes:


It isn't a matter of funding in this case. The country they are going to only allows a certain number of weapons in country with our people. The teams swap weapons as they rotate in and out of the base.

Our weapons are practically new since we are a guard unit and they sit in our armory most of the time with the exception of the guys who deploy fairly regularly(all of our deployments are filled by volunteers).

I know we joke about being able to fix anything with duct tape but that sucks about the 249.


When you say it would fire three rounds, then fail to fire, how many rounds total were fired? I am curious simply because I wonder if it really is a set pattern or simply an anomaly appearing to be a pattern.

Have you checked firing pin protrusion?
Head-space?
Action spring length?
What buffer weight?

I would swap the hammer spring with a new one just to be sure.
All I can recommend at this point is that you test-fire the weapon since the information you received is incomplete/lacking detail and all else seems to be in order. I would test-fire about 30 rounds slow fire, 30 rounds rapid fire, and 30 rounds burst.

I am interested to see what you discover during the test-fire.


The rifle is down currently out getting used for more pre-deployment training(lots of blanks probably). When it gets back I will see how it did and test fire and gauge it. I may go a head and replace the hammer spring just to be safe but it feels fine to me.

According to the SSgt who it is issued to she has only fired it for one qualification when it was new a few years ago and this last qualification where it was messing up. She has spent the time in between working on orders at another base. The rifle has less than 250 rounds through it.

I want to run it through some drills like you said to see how it does for me.

Thanks and I will let you know how it does when I get my hands on it again.

On the bright side, she was getting some good training on immediate action.

[nicholst55]

You doubtless already know this, but be sure to perform a complete function check before you start replacing parts. I personally detest the 3-round burst FCG; I've found that the parts seem to wear out inordinately quickly. Frequently you have to replace every moving part in order to get them running again - or so it seems to me.

[Iraqgunz]

Hopefully the weapons are receiving a proper pre-embarkation inspection. If there is any doubt about the reliability of the weapon I would either swap it with one that works for sure or I would get every part need to troubleshoot the weapon and fix it.


It isn't a matter of funding in this case. The country they are going to only allows a certain number of weapons in country with our people. The teams swap weapons as they rotate in and out of the base.

Our weapons are practically new since we are a guard unit and they sit in our armory most of the time with the exception of the guys who deploy fairly regularly(all of our deployments are filled by volunteers).

I know we joke about being able to fix anything with duct tape but that sucks about the 249.




The rifle is down currently out getting used for more pre-deployment training(lots of blanks probably). When it gets back I will see how it did and test fire and gauge it. I may go a head and replace the hammer spring just to be safe but it feels fine to me.

According to the SSgt who it is issued to she has only fired it for one qualification when it was new a few years ago and this last qualification where it was messing up. She has spent the time in between working on orders at another base. The rifle has less than 250 rounds through it.

I want to run it through some drills like you said to see how it does for me.

Thanks and I will let you know how it does when I get my hands on it again.

On the bright side, she was getting some good training on immediate action.