Worn out AR-15 bolt carrier or ?

[Luke E.]

 Okay, early this winter/ late fall one of my AR's that has been 100% reliable for 7,500+ rounds started becoming unreliable. Fast forward to now I have I think narrowed it down to the bolt carrier. Even once I new that it was something in the BCG I kept using it when I was out plinking and had time to mess with feeding failures so it has gotten worse. 

 Friday evening I went out behind the house with the questionable BCG (hereafter known as BCG1) and 2 more as well as 2 rifles and set up to pin this down on one part.

1.

- I tried the BCG1 and pulled the charging handle back and let it fly and it only got the round about halfway out of the mag.

- Next I slid a round into the chamber closed the bolt and slid a loaded mag in then fired.

- It ejected the spent case but only got the next round halfway into the chamber.

- With the round halfway in the chamber I hit the mag release and immediately upon the mag dropping it chambered the round that was previously hung up

- I tried 4 different mags (10rd Pmag, 20rd Pmag, 20rd GI and 30rd GI) and all had about the same results.

- The bolt carrier would about half the time ride up on top of the it was supposed to chamber jamming the case into the feed ramps and the rest of the time it would start picking the round up like it's supposed to but would only load the round halfway.

- With an empty mag inserted I would pull the charging handle back and let it go and the BCG moved forward very sluggishly and sometimes even completely unloaded it wouldn't even get the bolt to start locking up but as soon as I dropped the mag it would start sliding back and forth like it should

 

2.

- I moved BCG1 over to the 2nd rifle and had the same results all the way through.

 

3.

- I placed a different BCG in the 1st rifle and it functioned flawlessly as it did in the 2nd rifle.

 

4.

- I've swap bolts back and forth between carriers and cam pins between bolts etc... and the only combinations that have problems include BCG1

 

 It would seem that I should just accept that the carrier is worn out but i'm having a hard time understanding how a hardened steel carrier is going to wear out before the aluminum upper that it rides against. 

 

 Anyone have any other ideas or explanations as to what's going on?

 

 

 

 

 

 

 

 

 

[TrickyNicky]

How's the gas key look?

[Oh Shoot]

How's the gas key look?

 

Probably non sequitur since it is malfing just racking by hand, but yeah make sure it ain't all loose and wobbly of course.

 

Bottom bolt lugs that picks up next rounds look okay?

 

Try swapping just bolts as a variable.

 

edit: never mind, rereading,  I see you tried that apparently.

 

I too don't see how carrier wear could be the culprit, but I guess not all are made up to snuff possibly. Bottom of carrier look same as any others?

 

 

- OS

Edited April 15, 2014 by Oh Shoot

[Luke E.]

How's the gas key look?

 

 Gas key is fine but since it never makes it to firing status i'm not sure that it would matter if it even had one ;)

[Luke E.]

Probably non sequitur since it is malfing just racking by hand, but yeah make sure it ain't all loose and wobbly of course.

 

Bottom bolt lugs that picks up next rounds look okay?

 

 

- OS

 

 Only visible difference I can tell just by looking it that the bearing surfaces on the bottom are worn slick and shiny almost like they've been polished. The bolt lugs are fine and the bolt from the trouble BCG works fine when moved into any of the other carriers that I tried it in. I guess I need to get the calipers out and see what kind of difference I can find there. The fact that I can take an empty rifle and empty mag and insert the empty mag and the bolt carrier won't slide over the mag clean or even at all sometimes is what lead me to believe that the carrier is the issue. I would just think that it would have to wear a hell of a lot of material off the bottom of the carrier to cause this kind of problem.. Hopefully having more heads in the game will come up with an explanation though.

Thanks and keep'em comin'

[Oh Shoot]

With an empty mag inserted I would pull the charging handle back and let it go and the BCG moved forward very sluggishly and sometimes even completely unloaded it wouldn't even get the bolt to start locking up but as soon as I dropped the mag it would start sliding back and forth like it should

 

That's really bizarre part. If you hadn't  run through the other change-out variables with same rifle with success, I'd think misalignment with buffer/spring assembly somehow.

 

- OS

Edited April 15, 2014 by Oh Shoot

[Troutburger]

I never really thought of the carrier as being a wear item.

[Dolomite_supafly]

How hard is it to move the bolt into and out of the carrier compared to the others? Harder or easier?
 
A lot of crude/carbon can build up in the bottom of the carrier. This can prevent the bolt form completely locking up.
 
Something else, remove the gas rings and make sure to clean the channel they ride in. Wouldn't hurt to replace them if the gun have 7,500 rounds through them.

 

Have you inspected the cam pin? I have seen a lot of cam pins crack or become deformed long before 7,500 rounds.
  
With the bolt pushed all the way in the carrier is there any side to side movement when you push on the bolt? The carrier is chrome lined so I do not expect it to wear but outside of the bolt is not and it may. Or you could have worn the chrome lining out of the carrier. Could be that the gas rings and bolt itself are worn so much that when trying to strip the round from the magazine the pressure on the front of the bolt is causing the cam pin to lock the bolt up. Basically when pushing on the front of the bolt it forces it back and that causes the cam pin to exert sideways pressure locking the bolt carrier up.

Here is a video about the cam pin lock up.
https://www.youtube.com/watch?v=6JmIQXkoog8

[Luke E.]

How hard is it to move the bolt into and out of the carrier compared to the others? Harder or easier?
 
A lot of crude/carbon can build up in the bottom of the carrier. This can prevent the bolt form completely locking up.
 
Something else, remove the gas rings and make sure to clean the channel they ride in. Wouldn't hurt to replace them if the gun have 7,500 rounds through them.

 

Have you inspected the cam pin? I have seen a lot of cam pins crack or become deformed long before 7,500 rounds.
  
With the bolt pushed all the way in the carrier is there any side to side movement when you push on the bolt? The carrier is chrome lined so I do not expect it to wear but outside of the bolt is not and it may. Or you could have worn the chrome lining out of the carrier. Could be that the gas rings and bolt itself are worn so much that when trying to strip the round from the magazine the pressure on the front of the bolt is causing the cam pin to lock the bolt up. Basically when pushing on the front of the bolt it forces it back and that causes the cam pin to exert sideways pressure locking the bolt carrier up.

Here is a video about the cam pin lock up.
https://www.youtube.com/watch?v=6JmIQXkoog8

 

The bolt moves in and out smoothly (looser than a brand new one but not any more so than one I have with 600 rounds on it)

 

There is no carbon anywhere on/in the carrier or bolt. All have been soaked in kroil for 24hrs and carbon removed with a tool designed to access all critical points of a BCG. I stuck a bore light in the carrier and verified this before the testing.

 

Gas rings were replaced at approximately 5,000rds but the ring groove is clean now.

 

The cam pin has also been replaced and was replaced again during testing.

 

The bolt does have a small amount of play in the carrier but I don't have any of my other newer groups with me to check it against (just happened to have the questionable one sitting in my truck. 

 

 While there may be excessive play between the bolt and carrier bore but it really wouldn't explain the fact that it still doesn't function correctly with zero rounds of ammo within reach. Like I mentioned above, I can insert an empty mag into an unloaded rifle (no ammo in the equation) and the BCG still drags and it seems like the front of the bolt is rubbing across the top of the mag. but when I remove the empty mag it cycles as it should. I also took known functioning bolts and installed them in this carrier and had the problem as well as I took the questionable bolt and installed it into other carriers and had no problems. 

 The one thing I did not try was to remove the bolt from the carrier and install only the carrier into the rifle. After all the options I tested and the results I got I would suspect that I would still have the problem with no bolt installed in the carrier with an unloaded mag inserted in the rifle. I will check this when I get home tonight. I think the components of this BCG group likely worn with 7,500+ rounds on them and in the name of reliability I should probably replace it anyways but I sure would like to know what is causing the actual failures i'm experiencing just to satisfy curiosity.

 

Thanks for the lending of brain power folks!

[Dolomite_supafly]

And this was with a gun that functioned fine for thousands of rounds? Didn't replace the lower?

[Luke E.]

Nothing has been changed with the rifle. And to top that off I can take the bad BCG and slide it into other known functioning rifles and it gives the same problems. It's my wife birthday today but if I get time to I will try to capture the problem on video tonight and post it.

[MacGyver]

I'm guessing you have some excessive wear on either or both the inside of the bolt carrier and the exterior of the bolt itself as evidenced by the slop as you move it by hand.  Add a little bit of upward pressure from the round in the magazine and it kicks out of tolerance just far enough that it's binding.  That's why if you drop the magazine and/or rack it by hand without a magazine in place it'll appear to do just fine.  I'd bet that if you were to take a micrometer to the separate pieces you'd find #1 out of tolerance.

[Luke E.]

I'm guessing you have some excessive wear on either or both the inside of the bolt carrier and the exterior of the bolt itself as evidenced by the slop as you move it by hand. Add a little bit of upward pressure from the round in the magazine and it kicks out of tolerance just far enough that it's binding. That's why if you drop the magazine and/or rack it by hand without a magazine in place it'll appear to do just fine. I'd bet that if you were to take a micrometer to the separate pieces you'd find #1 out of tolerance.

I'd say you're right about the wear but it seems odd that it still does this even when there are no rounds in the rifle or mag. Maybe it's the fact that each part is worn so when I split the parts up some will function. I'll do some measuring and comparing of each and see what I get.

[c.a.willard]

A warped gas key can cause your issues as well and would cause binding with even the slightest upper pressure.  I would start there.  The odds that your carrier itself has gone bad after functioning fine for that many rounds is pretty slim.  It is also cheaper than buying a new carrier.  Try pulling the key and then hand cycling to see if you are still getting the issues.  

Edited April 15, 2014 by c.a.willard

[Luke E.]

A warped gas key can cause your issues as well and would cause binding with even the slightest upper pressure. I would start there. The odds that your carrier itself has gone bad after functioning fine for that many rounds is pretty slim. It is also cheaper than buying a new carrier. Try pulling the key and then hand cycling to see if you are still getting the issues.

That was the point in me starting this thread. Everything I was doing was pointing at a part that I would consider to be one of the toughest parts in the rifle so I just needed fresh eyes on it. I think I'll pull the bolt out and try just the carrier and if that still gives the problem I'll pull the gas key. The main reason I doubt it's the keys is that I pulled the upper and and the bolt to check gas tube alignment and that all seems fine. Second it the fact that it doesn't matter if I'm actually firing the gun or just run the action by hand with no ammo loaded. It's not that I don't have another BCG to throw in it because I have that, it's mainly just to satisfy my curiousity of wanting to know what is causing it not to work. I'll definitely check the key to make sure though. Thanks!

[molonlabetn]

Sounds like a weak buffer spring. I've had rifles do that when overgassed or when the buffer spring is weak or the buffer was too light. Why it's only doing it with one of the carriers is strange... Are the carriers identical? Edited April 16, 2014 by molonlabetn

[Luke E.]

Sounds like a weak buffer spring. I've had rifles do that when overgassed or when the buffer spring is weak or the buffer was too light. Why it's only doing it with one of the carriers is strange... Are the carriers identical?

 

Rifle works fine when I put a different BCG in it so the buffer spring is safe.

[Dustbuster]

Not trying to be a smart but after reading through this,if your confident that it's a bcg issue,I'd grab a new one and bench the bad one.if you're running a hefty amount of ammo, why waste any more time or risk catastrophic damage. Get a spikes or JP bcg and keep running it. I'm also assuming your FCG and other Lower parts are gtg


Sent from my iPhone using Tapatalk 2 of course it ate my spelling.

[Luke E.]

Not trying to be a smart but after reading through this,if your confident that it's a bcg issue,I'd grab a new one and bench the bad one.if you're running a hefty amount of ammo, why waste any more time or risk catastrophic damage. Get a spikes or JP bcg and keep running it. I'm also assuming your FCG and other Lower parts are gtg


Sent from my iPhone using Tapatalk 2 of course it ate my spelling.

I have a couple of new ones so that's not the issue. I just like to know what exactly went wrong. Anytime I want to go to the range or shoot without fooling around with that issue I just throw another one in it.
I've had zero time off so I've still not checked the thing any further.

[Dustbuster]

I hear ya, stupid question u running your bolts wet or quasi dry?


Sent from my iPhone using Tapatalk 2 of course it ate my spelling.

[Dolomite_supafly]

Running them dry will not cause this problem. Running dry might eventually cause other problems but not picking up a round out of the magazine is not a lube issue.

 

Can you post pictures of the bolt, not the carrier? Specifically a 3/4 frontal shot and maybe an overall shot.

[Luke E.]

I hear ya, stupid question u running your bolts wet or quasi dry?

It gets oiled but as you know, when go out and run 500 rounds it is pretty well dry after that.

Sent from my iPhone using Tapatalk 2 of course it ate my spelling.

[Luke E.]

Running them dry will not cause this problem. Running dry might eventually cause other problems but not picking up a round out of the magazine is not a lube issue.

Can you post pictures of the bolt, not the carrier? Specifically a 3/4 frontal shot and maybe an overall shot.

I can when I get home and have a minute. My sisters wedding is this weekend and the rehearsal dinner is at our place so have been running until 9 most nights trying to get things done. That's why I've not done anything else with this.

[TMF]

Wish I had it in my hands so I could answer my own questions.  This is something unique.  BCGs don't just "wear out" to where they bind up when chambering a round.  That doesn't make any sense.  There are plenty of AR rifles that have BCGs with hundreds of thousands of rounds through them that continue to work just fine despite the BCGs looking like they were dragged behind a jeep for a few miles.  Something else is going on here.

 

I'm gonna go with inspecting the cam pin and where the bolt goes into the carrier.  That's the only logical thing I can think of without having it in my hands.  That cam will build up carbon that is tough as steel and so will the inside of the carrier where the bolt inserts.  I've had to take that garbage off with a 249 tool before, but never saw it so bad that it prevented the thing from chambering a round.  I dunno, scrape that stuff off and see how it goes.  Could you post a picture of the cam as well as the inside of the carrier where the bolt is housed?

 

ETA: Did not see that this went on to a second page and Dolo already asked.

Edited April 21, 2014 by TMF

[Luke E.]

Wish I had it in my hands so I could answer my own questions.  This is something unique.  BCGs don't just "wear out" to where they bind up when chambering a round.  That doesn't make any sense.  There are plenty of AR rifles that have BCGs with hundreds of thousands of rounds through them that continue to work just fine despite the BCGs looking like they were dragged behind a jeep for a few miles.  Something else is going on here.

 

I'm gonna go with inspecting the cam pin and where the bolt goes into the carrier.  That's the only logical thing I can think of without having it in my hands.  That cam will build up carbon that is tough as steel and so will the inside of the carrier where the bolt inserts.  I've had to take that garbage off with a 249 tool before, but never saw it so bad that it prevented the thing from chambering a round.  I dunno, scrape that stuff off and see how it goes.  Could you post a picture of the cam as well as the inside of the carrier where the bolt is housed?

 

ETA: Did not see that this went on to a second page and Dolo already asked.

 

I dropped a new cam pin in it and it didn't seem to make any difference. I've kept the carbon cleaned out of the carrier bore where the bolt slides in and before I went out and fooled with it last I soaked it and took the scraping tool to it and it is pretty well spotless. I agree that it shouldn't be worn out after so few a numbers of rounds and part of me keeps thinking that i'll find something silly that is the cause. I'm not making it home tonight so it'll be at least tomorrow night before i'm back around it again. As far as one looking like it's been dragged behind a truck this one will disappoint you because it really doesn't look that bad.