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Major Malfunction


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#1 StrangeRanger

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Posted 07 August 2019 - 01:11 PM

Miss Piggy took a major dump on Sunday.  In the middle of my second 100 rd belt I had not one but three case head separations.  Ammo was GGG (Lithuania) headstamped with the NATO cross symbol and it seems to be highly regarded.  I could find no reports of case failures with it on line. So if I assume it's not an ammo issue, what can cause a 60 to suddenly develop excessive headspace? Or to fire out of battery?

 

 

The locking recesses on the barrel extension are unharmed.

 

The bolt has the traditional chip out of the left front corner of the top/right lug but the rear faces of both lugs are good.  The guide surfaces show some wear and parkerizing is worn on the large diameter of the bolt body in a lengthwise stripe behind the top lug indicating it is rubbing on the trunnion when the bolt goes into battery.

 

The front radius of the drive lug on the op rod has seen better days; the ridges and burrs have been stoned smooth but this was going to be its last trip to the range.  The firing pin drive lugs show some wear but I'd generally rate them as good.  All the guide surfaces are intact and show no excessive wear.  There is a wear pattern in the parkerizing on the flat on the bottom behind the sear notch but no other damage.  (Speaking of which, one of my spare op rods, the oldest one, does not have this flat, the others do.  Does this matter?)

 

I can see no significant damage to the receiver.  The trunnion is solid and the welds all appear to be intact.  There is very minor wear on the front of the top right and lower left corners of the bolt socket where the bolt has rubbed.

 

I'm pretty much at a loss here, what am I missing?  Absent a definable cause, I'm planning on putting in a new bolt and op rod (which I was going to do anyway after this trip to the range) and switching to a belt of known quality (Prvi Partisan) ammo and seeing what happens.  But I'd really like to suss this one out before I do that.


Edited by StrangeRanger, 07 August 2019 - 03:24 PM.

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#2 mattnh

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Posted 07 August 2019 - 08:45 PM

Qs:

1) Same combination of barrel and bolt as before or perhaps you recently swapped barrels?

2) Have you checked headspace with a no-go or field headspace gauge?

3) Extractor look OK?


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#3 StrangeRanger

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Posted 07 August 2019 - 09:16 PM

1) Same barrel/bolt package as before.  The bolt and op rod have now been swapped out for  new ones but not fired yet.  I have a second barrel but I did not swap it just yet.

 

2) Haven't checked but there's never been a problem till this week.  

 

3) The extractor is fine.  Looking at the head of one of the stuck cases, it looks like the case may have gotten stuck in the chamber and the extractor literally tore the head off, lots of damage to the extractor groove but none to the extractor.  The case came out with an expanding mandrel type stuck case remover.  The barrel was relatively clean when I started shooting Sunday and I had put 150± rounds through it before the malfunction so I'm not sure why the case would have gotten stuck if that is what did happen.


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#4 mattnh

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Posted 07 August 2019 - 10:50 PM

What did the broken cases look like when you got them out? Were the case necks split?

 

Did you look over all the other spent brass which didn't get stuck?  Did you notice anything abnormal like split necks?

 

How did the primer strikes look on the spent brass?  Light/Normal/Deep?


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#5 StrangeRanger

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Posted 08 August 2019 - 05:07 AM

Case necks on the stuck cases were intact, no splits.  Nothing remarkable about the cases that did not stick.

The primers shower normal strikes, the same as from previous shoots.  I dug some out of the scrap bin to compare.

The separated case heads were the last 3/8" of the case, basically right where the walls transition from the inside taper to a constant thickness.


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#6 mattnh

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Posted 08 August 2019 - 09:00 AM

It certainly has a symptom of excessive headspace, but I wouldn't say it is 100% definitive.

 

It is a really good idea to check headspace between all possible combinations of barrel and bolt you plan to use.

Just because a barrel and bolt each are in-spec does't mean the combination of the two are OK to use.

Swapping bolts/barrels around w/out checking HS usually works OK, but it can end in disaster... 


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#7 TSMGguy

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Posted 08 August 2019 - 09:45 AM

Sounds like the cases are sticking in the chamber after obturation, for whatever reason. The cases are not extracting, and the extractor is tearing the heads off of the cases. Is there any roughness in the walls of the chamber? Any interior corrosion where the case heads meets up with the rest of the case?  


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#8 StrangeRanger

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Posted 08 August 2019 - 10:30 AM

Sounds like the cases are sticking in the chamber after obturation, for whatever reason. The cases are not extracting, and the extractor is tearing the heads off of the cases. Is there any roughness in the walls of the chamber? Any interior corrosion where the case heads meets up with the rest of the case?  

That would be my best diagnosis at this time as well.  However, there are no rough spots or corrosion in the chamber that I can see.  There may have been some minor carbon buildup from about 350 rounds fired since last scrubbing but that's never been an issue before.  Basically I can find nothing in the gun that caused the problem. 

 

A friend suggests that a NATO headstamp doesn't guarantee that there are zero bad rounds, so maybe it is a problem with too soft brass but that seem so unlikely that I hesitate to settle for it as an answer.

 

I think I'm going to take it back to the range this afternoon and put a belt of Prvi through it and see if it runs 100%; Prvi has always cycled perfectly and leaves very little residue.  If it does then I'll put a belt of GGG through it to see if it has a repeat failure.


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#9 mattnh

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Posted 08 August 2019 - 01:36 PM

BTW,

I seem to recall in one of Aaron's? M60 videos he mentioned something about corrosion in barrel chamber

after just sitting for a short while causing problems...

The video showed using an electric drill with a nylon brush to super-clean the chamber...


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#10 mattnh

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Posted 08 August 2019 - 02:07 PM

One thing to check:

Remove barrel in question from gun

Does cartridge drop freely into chamber and fully seat? - it should

If you turn barrel up, does the cartridge freely drop out? - it should


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#11 StrangeRanger

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Posted 08 August 2019 - 02:40 PM

One thing to check:

Remove barrel in question from gun

Does cartridge drop freely into chamber and fully seat? - it should

If you turn barrel up, does the cartridge freely drop out? - it should

I did and it does.


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#12 mattnh

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Posted 08 August 2019 - 02:56 PM

Hmmm - Sounds like chamber isn't rough...

 

 

Best guess - some combination of the following:

  Excessive Headspace

  Overgassed extraction (is the barrel old/shot-out - eroded gas port?)

  Weak brass / bad ammo


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#13 StrangeRanger

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Posted 08 August 2019 - 03:46 PM

The barrel has seen quite a bit of use but the rifling is still strong so I never thought about the gas port.  I will have to check.

At the moment I'm about to leave for the range to do a function test with the new bolt & op rod and both known good and the somewhat suspect ammo from Sunday


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#14 mattnh

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Posted 08 August 2019 - 07:20 PM

The barrel has seen quite a bit of use but the rifling is still strong so I never thought about the gas port.  I will have to check.

At the moment I'm about to leave for the range to do a function test with the new bolt & op rod and both known good and the somewhat suspect ammo from Sunday

 

I was going to ask if the barrel was a newer one with adjustable gas  (maybe you got it cranked-up to max (i.e. way overgassed if clean)?)


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#15 StrangeRanger

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Posted 08 August 2019 - 09:16 PM

Well I just got home from the range and I'm 99% certain it was an ammo issue perhaps abetted by a less than perfectly clean chamber.

 

I ran a 100 round belt of Prvi with zero issues as expected then without allowing the gun to cool like I usually would, I ran 100 of the GGG.  Again no issues.  However a post-mortem inspection of the fired cases showed some differences.  The GGG showed .001-.002 more expansion at the critical point, 3/8" above the head where the failure occurred.  The GGG cases had a very distinct scuff line at that point and were bright and unscuffed below it; the Prvi cases showed a more general scuffing along the entire length and no distinct line.  Primers looked identical with no signs of cratering so it is not an overpressure issue.  Cases weighed the same so thin walls are unlikely.  I'm guessing it's simply very soft brass and possibly a few that are WAY too soft.  If there's any significant crud in the chamber, and there usually is with any gas gun, the cases can stick just enough to cause problems.  I'm guessing that swabbing the chamber with mineral spirits between belts until I run out of GGG will solve the problem. 

 

Or not.

 

GGG on left, Prvi on right

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