Help w/ intermittent FTE/stovepipe issue

[Barnacle Bill]

My newest has a BCM 14.5" barrel (w/ permed A2X FS), Colt BCG, and what started as a complete Colt LE6920 lower (I installed a new trigger & buttstock). The bolt came with a black extractor spring insert, no o-ring. The lower came with an H buffer, and the buffer spring measures right at 10.5" long when relaxed.

So far I have 176 rounds through it. Twice I've had an FTE/stovepipe, with the next round partially chambered and getting dented in the process. Both times were with PPU M855, which was 60 rounds of the 176 round total (the first time I've used that particular ammo, although I've shot a lot of other PPU stuff before). Each time was the first round in a mag that had been inserted less than completely full (9 rounds in one, 20 in the other), both PMAG Gen3 30-rounders. 100 rounds of what I fired was Mk 318 (Fed T556TNB1 "white box"), but no problems with that (or with 16 rounds of old Norinco stuff I bought at a gun show about 25 years ago). So far the problem was only with the PPU.

The carbine was lubed the same way I do with my other trouble-free AR's, and everything was still wet on disassembly so I don't suspect a lube issue.

This is, however, my first AR with less that a 16" barrel.

The PPU has green paint on the bullet & pretty large amounts of red primer sealant, and I find green & especially red flecks all over the action - even on the part of the firing pin that is completely inside the bolt, and in the channel in the bolt under the extractor.

So, is this just the PPU, or do I need to install an extractor spring upgrade/o-ring or even a different buffer?

[Eric D.]

Just to clarify are you having a stovepipe malfunction like this:

http://i.imgur.com/2kX2dDo.jpg

Or a bolt over base malfunction like this? Or both?

http://i.imgur.com/MCgCyX6.jpg

[Dead Man]

Sounds like your ejector's struggling. Maybe you got some of that paint and sealant flaking crudded up in there. It's kind of a pain in the ass to pull an ejector without an ejector tool, so if you don't have one, I'd try running a different bolt and see if you get the same problem.

[bluejackets92fs]

Try it with out Privi ammo and see if it repeats.

[Barnacle Bill]

Just to clarify are you having a stovepipe malfunction like this:

http://i.imgur.com/2kX2dDo.jpg



Yep, that's it.


Sounds like your ejector's struggling. Maybe you got some of that paint and sealant flaking crudded up in there. It's kind of a pain in the ass to pull an ejector without an ejector tool, so if you don't have one, I'd try running a different bolt and see if you get the same problem.

Possibly, although it seems pretty strong when I depress it with a tool.

I could drop in another bolt easy enough.


Try it with out Privi ammo and see if it repeats.

Yeh, that stuff is highly suspect. The only other 62gr stuff I have is the Mk 318, which is currently unobtanium and was too $$$ to waste before that. I have some XM193 & Q3131a I can try, or I could pick up some XM855 as that has resurfaced on the market at not too dear a price (by contemporary standards).

[bluejackets92fs]

All my M855 is Federal stuff that came in the 420 round 30 cal cans. It has been fine but out of my BCM accuracy at 50 yards is not as good as XM193 Federal.

[Barnacle Bill]

All my M855 is Federal stuff that came in the 420 round 30 cal cans. It has been fine but out of my BCM accuracy at 50 yards is not as good as XM193 Federal.

Does it have the green paint on the tip, and if so does it flake off in your rifle? I'm seeing a lot more red from the primer seal than green from the tip paint, but there is definitely some green (not down inside stuff like the red, either). If Mk 318 was as cheap & available as M855, I guess I'd be golden, but if wished were fishes things would be real different in Washington;)

[Tzed250]

The Federal M855 that I have been shooting(20rd boxes in the 420rd can) has left zero green paint inside my action.

[Tzintzuntzan]

M855 is nicknamed "green tip" for a reason. Paint flecks will not cause these problems in your rifle, something else is the culprit.

[bluejackets92fs]

Does it have the green paint on the tip, and if so does it flake off in your rifle? I'm seeing a lot more red from the primer seal than green from the tip paint, but there is definitely some green (not down inside stuff like the red, either). If Mk 318 was as cheap & available as M855, I guess I'd be golden, but if wished were fishes things would be real different in Washington;)

I've never found paint in my chamber. Priv will leave my bolt face red aftet a mag though but never has it caused issues.

[Todd.K]

A carbine gas 14.5" would benefit from an H2 buffer in my opinion. Especially with 5.56 pressure ammo.

What extractor spring? How many rounds on it? Make sure it's the latest Colt carbine spring, if it's a rifle spring add an O-ring.

[T2C]

Just to clarify are you having a stovepipe malfunction like this:

http://i.imgur.com/2kX2dDo.jpg

Someone at work had intermittent issues like this. I inspected the bolt and the ejector was dragging in the bolt. I removed the ejector and found a small metal chip in the ejector hole. I completely disassembled the bolt, cleaned and lubed all the parts and reassembled the bolt. That was all I needed to do to correct the issue.

[Barnacle Bill]

A carbine gas 14.5" would benefit from an H2 buffer in my opinion. Especially with 5.56 pressure ammo.

What extractor spring? How many rounds on it? Make sure it's the latest Colt carbine spring, if it's a rifle spring add an O-ring.

The BCG came complete from Specialized Armament, advertised as "BOLT GROUP, LE/MILITARY, COLT FACTORY TEST FIRED". Bolt carrier is stamped "C" on the left side. Key appears properly staked. Gas rings look new & shiny.

Extractor spring has the black insert, no o-ring. Some of that red debris was even inside the extractor channel in the bolt.

Other than the insert color, I couldn't tell a carbine extractor spring from a rifle extractor spring. I've been thinking about getting the BCM bolt rebuild kit that I think is based on the USGI Crane reliability kit (HD extractor spring, new black insert, Crane o-ring, new set of standard gas rings just in case). However, see my next post.

[Barnacle Bill]

Someone at work had intermittent issues like this. I inspected the bolt and the ejector was dragging in the bolt. I removed the ejector and found a small metal chip in the ejector hole. I completely disassembled the bolt, cleaned and lubed all the parts and reassembled the bolt. That was all I needed to do to correct the issue.

Well, this has been my unlucky weekend for AR's. I have this Colt 6920 that I bought NIB about 4 years ago and until this month left it bone stock - whatever it came with from the Colt factory except an assortment of slings (on the factory swivels) and BUIS & optics (on the factory upper's top rail). Until I messed with it this month, it was always 100% reliable with a wide assortment of ammo, including on one occasion the year I got it of over 800 rounds of Win Q3131a without cleaning or relubing (started out lubed with Tetra grease) over the course of a 3-day carbine class.

So, I figured I'd swap the bolt from old faithful into the new LW build, as discussed above. However (a) I'd never fired any 62gr in it before, only 55gr & 75-77gr, and (b) I decided to convert it to a SOPMOD block IIish build so I sent it to ADCO to get...
...barrel shortened to 14.5", threaded, M4 profile and BCM A2X permed
...DD RIS II FSB installed as he put it back together

So, I figured I would check out the newly-modified 6920 and in the process zero it with the Mk 318 before doing any bolt swap testing. Unfortunately that didn't work out so well. The first 5 rounds were the last of this old Norinco 55gr stuff left over from c. 1990, to make sure I was on paper. Then 100 rounds of Mk 318. I was also checking out 4 new PMAG Gen3's with the Mk 318, so I was loading one round from another mag then installing a full 30 round mag and firing with that (off & on while zeroing BUIS & and ACOG) until the bolt locked back on an empty mag. Then repeat with another mag. Three mags - no problem. By that point I'm into the last 10 of the 100 rounds of Mk 318 and it looks good. On the 4th, with the last of the Mk 318 on top (plus one in the chamber) and the rest of the 30 rounds in the mag filled up with the PPU, it short strokes. I repeated this 3 or 4 times, and initially blamed the mag. Then I switched back to mag #3 that had passed the test a few minutes ago - still short stroking. Then I got down into the mag and in fact fired 30 rounds total of the PPU, with intermittent short-strokes all the way down the mag.

By short stroke, I mean the bolt is heading for closed without picking up the top round in the mag. I was definitely checking that the mag was locked in. Sometimes when this happened the bolt wouldn't even reach lock-up, apparently hanging up from the friction against the top round in the mag.

So, now I have to fix this problem before I can go back to working the LW build. I think that's the right order, because the 9620 worked perfectly until I messed with it (not that I can really go back short of a new barrel). So, I'm going to thoroughly clean & lube it, then go back to ammo I know functioned perfectly in it before (my remaining stash of Q3131a and the now readily-available XM193) and take it from there.

It's enough to make me snatch myself bald:mad:

[steyrman13]

So the only thing causing issues in both rifles is the PPU 62gr? Sounds like there's your problem....

[Iraqgunz]

I doubt cleaning it is going to do anything for your situation.

1. Use an AR that hasn't been dicked with at ALL and then fire the suspect ammo. If it malfunctions then you can start to blame the ammo. I would try the lock back test first for about 10 rounds.

2. If it functions then I would go back and look at the other AR. Since we know work was done you need to check the gas tube, FSB and make sure that the carrier key is torqued and staked.

3. I would stop using grease. JM2CW.


Well, this has been my unlucky weekend for AR's. I have this Colt 6920 that I bought NIB about 4 years ago and until this month left it bone stock - whatever it came with from the Colt factory except an assortment of slings (on the factory swivels) and BUIS & optics (on the factory upper's top rail). Until I messed with it this month, it was always 100% reliable with a wide assortment of ammo, including on one occasion the year I got it of over 800 rounds of Win Q3131a without cleaning or relubing (started out lubed with Tetra grease) over the course of a 3-day carbine class.

So, I figured I'd swap the bolt from old faithful into the new LW build, as discussed above. However (a) I'd never fired any 62gr in it before, only 55gr & 75-77gr, and (b) I decided to convert it to a SOPMOD block IIish build so I sent it to ADCO to get...
...barrel shortened to 14.5", threaded, M4 profile and BCM A2X permed
...DD RIS II FSB installed as he put it back together

So, I figured I would check out the newly-modified 6920 and in the process zero it with the Mk 318 before doing any bolt swap testing. Unfortunately that didn't work out so well. The first 5 rounds were the last of this old Norinco 55gr stuff left over from c. 1990, to make sure I was on paper. Then 100 rounds of Mk 318. I was also checking out 4 new PMAG Gen3's with the Mk 318, so I was loading one round from another mag then installing a full 30 round mag and firing with that (off & on while zeroing BUIS & and ACOG) until the bolt locked back on an empty mag. Then repeat with another mag. Three mags - no problem. By that point I'm into the last 10 of the 100 rounds of Mk 318 and it looks good. On the 4th, with the last of the Mk 318 on top (plus one in the chamber) and the rest of the 30 rounds in the mag filled up with the PPU, it short strokes. I repeated this 3 or 4 times, and initially blamed the mag. Then I switched back to mag #3 that had passed the test a few minutes ago - still short stroking. Then I got down into the mag and in fact fired 30 rounds total of the PPU, with intermittent short-strokes all the way down the mag.

By short stroke, I mean the bolt is heading for closed without picking up the top round in the mag. I was definitely checking that the mag was locked in. Sometimes when this happened the bolt wouldn't even reach lock-up, apparently hanging up from the friction against the top round in the mag.

So, now I have to fix this problem before I can go back to working the LW build. I think that's the right order, because the 9620 worked perfectly until I messed with it (not that I can really go back short of a new barrel). So, I'm going to thoroughly clean & lube it, then go back to ammo I know functioned perfectly in it before (my remaining stash of Q3131a and the now readily-available XM193) and take it from there.

It's enough to make me snatch myself bald:mad:

[Barnacle Bill]

So the only thing causing issues in both rifles is the PPU 62gr? Sounds like there's your problem....

No, the 6920 started short-stroking on the Mk 318 before I ever got to the PPU.

[steyrman13]

No, the 6920 started short-stroking on the Mk 318 before I ever got to the PPU.

I would do what IG says first.

Is it possible that you lost count and the last 2 rounds of Mk 318 were in fact the PPU as well?
The PPU is definitely giving you issues on both weapons for sure. The MK318 made it 98 before failure or possibly miss counted 2 rnds and no failures.
Also as IG said use quality lube not grease

[Barnacle Bill]

I doubt cleaning it is going to do anything for your situation.

I tend to agree, except maybe eliminate anything implicating the ejector per T2C's suggestion. The 6920 has thousands of rounds through it and while I routinely remove the extractor for cleaning I've never had the ejector out. Crud in there wouldn't cause the short-stroking, at least I can't think of how it could, but it could relate to the stovepipe issue on the new LW build. Anyway, on the 6920, going back to ammo it used to work with seems a reasonable step, and starting that test freshly cleaned & lubed just eliminates one more variable. Now that I think about it, my first AR build c. 1990 initially had some sort of problem (details of the symptoms escape me after nearly 25 years) that turned out to be as simple to fix as running a pipe cleaner through the gas key. Clean couldn't hurt.


1. Use an AR that hasn't been dicked with at ALL and then fire the suspect ammo. If it malfunctions then you can start to blame the ammo. I would try the lock back test first for about 10 rounds.

Fortunately, this I can do. I have a Colt 6520 that hasn't had any work done on it except by Colt (out of the box the feed ramps in the upper were not aligned with those in the barrel extension, they fixed it under warranty). It doesn't have anywhere near the round count of the 6920, but never had any issues feeding & ejecting.


2. If it functions then I would go back and look at the other AR. Since we know work was done you need to check the gas tube, FSB and make sure that the carrier key is torqued and staked.

The LW upper from the OP was assembled by a local smith who builds a lot of AR's and has a lot of repeat customers. He had to mount the FSB onto the barrel and install the gas tube. All those parts were new. It is possible something is wrong, but I can't find anything loose or obviously misaligned. As the barrel is unmodified from the way it was made, I'm assuming the gas port is the right size for that length. The BCG was a new Colt, and all the local smith did with it was drop it in and test fire it. Gas key appears properly staked and I can't detect any looseness.

On the 6920, ADCO would have had to remove the FSB & gas tube to install the rail system. Again it is possible something is wrong, but I can't find anything loose or obviously misaligned. It could be the gas port is smaller than ideal for a 14.5" barrel, since it was made as a 16", but I would have expected a highly experienced shop like ADCO to raise that issue if it was commonly a problem (I get the impression they shorten a lot of barrels). The factory BCG is stock,
gas key appears properly staked and I can't detect any looseness.

So, if the 6520 runs the new ammo like a champ, I'm probably going to need professional help to sort out the two bad boys.



3. I would stop using grease. JM2CW.

Care to elaborate? As I say, I've had great luck with it over the years. Its pretty hard to imagine a LW grease would cause the 6920 to start short-stroking after 100 rounds as a 14.5" when the same gun ran over 800 rounds on one application of the same grease as a 16". However, I'm open to being learned better.

[steyrman13]

No, the 6920 started short-stroking on the Mk 318 before I ever got to the PPU.

In the OP the only ammo that caused issues with the LW build was the PPU as well...

[Barnacle Bill]

Is it possible that you lost count and the last 2 rounds of Mk 318 were in fact the PPU as well?
The PPU is definitely giving you issues on both weapons for sure. The MK318 made it 98 before failure or possibly miss counted 2 rnds and no failures.


No, I when I went to PMAG #4 there were exactly 7 rounds of Mk 318 left. 6 went on top of 24 rounds of PPU, and the 7th went in a different mag to load the chamber before inserting #4. The first thing I did was remove the mag & all the rounds in it to count & make sure I hadn't somehow stuffed 31 in there. It was 30. Then 25 rounds of PPU went in under 5 rounds of Mk 318, and one 318 in the other mag. Then I repeated the whole unload & count & reload thing again once or twice. Then after taking all the rounds out I switched to PMAG #3 that had worked fine just before I took out #4, only now it did it too. However, when #3 first went south it still had one Mk 318 up the spout and at least one on top in the mag.

[Barnacle Bill]

In the OP the only ammo that caused issues with the LW build was the PPU as well...

Very true, and it was a big surprise that the 6920 started acting up before I got to the PPU. Nor did it act up the same way, as I never saw any stovepipes on the 6920.

[Barnacle Bill]

OK, based on getting under the hood on the 6920 I'm halfway there on the grease thing.

I'll preface by stating that Tetra is not like the old school military rifle grease. It's a lot closer to liquid than that. Also, in addition to going over 800 rounds on a single application of it in the 9620, I also once went about 500 in a CZ P-01 on a 3 month old application of it, and over 1600 in a Beretta 92 over the course of a couple months on a single application of it. The directions on the package say to buff it dry, but I've always left a light coat looking wet (as grease would traditionally be used). Until now, this has worked like a champ in all my semi-autos.

That said, though, the 9620 sat much longer than any of the above. I wiped out the inside of the upper before sending it to ADCO and reapplied Tetra to that after it came back. However, I just took out the BCG before shipping the upper and put it in a plastic bag to wait (it was only 6 days - ADCO was impressively fast). Because it still looked wet, I just dropped the BCG back in and went to the range.

Cleaning it, I found the BCG unusually "gummy". I've cleaned lots of BCG's that had been lubed with Tetra and then fired, and they usually just clean right up with a few squirts of gunscrubber & maybe a little toothbrush action, then a wipedown with CLP for rust prevention. This took a lot more solvent & elbow grease. So, I think what happened is the Tetra set up over time and the 6920 was acting more or less like it was dry, despite the Tetra looking wet.

Hopefully Thursday I'll get the following done...

Test the two new ammos (Mk 318 & PRVI M855) in the bone-stock 6520, starting with the mag lockback test (and the carbine freshly cleaned & lubed to eliminate that variable).

Test the 6920 (freshly cleaned & lubed) with known good ammo, again starting with the mag lockback test. I'll fire XM193 & Q3131a until I've gone 200 or run out or hit big trouble again, then if it looks good try the new ammos again.

While I have both BF CLP and Slip 2000 (which I've never used yet) on hand, to not introduce any new variable I'm going to stick with the Tetra for these tests (just freshly applied after thorough cleaning). I'm thinking that once I get through this, I'll try the SLIP 2000 in the 6920.

If this works out, then I can get back to figuring out why the new LW build is stovepiping.

[steyrman13]

Get some non-chlorinated brake cleaner and clean the tetra out of the entire upper and BCG. Then lube with the slip 2000 test. It is a known performer and by re using tetra you will be wasting ammo and time again.

[T2C]

It sounds like you are on the right track. I agree with using non-chlorinated brake cleaner and thoroughly cleaning the upper.

I would use some kind of oil, Break Free CLP, Mobil 1 motor oil, etc., to test the carbine after cleaning. If the issue is resolved, you can always test the more modern lubricants later.

[Barnacle Bill]

The new synthetic-safe Gunscrubber is functionally equivalent to non-chlorinated brake cleaner, but several cans of it currently reside in my basement.

I guess I can try the Slip 2000 for the tests, although that becomes a new variable. It may be a known performers generically, but I never used any before myself (bought it back in the winter after seeing on a video how well it does in sub-zero temperatures, but never got around to trying it before the weather warmed up).

[Barnacle Bill]

OK next installment...

The 6520 (with the grease cleaned out & relubed with Slip 2000 EWL) digested 100 each of the Mk 318 & the PPU M855, without any trouble at all. Part of that was doing the lock back test with a mag with only one round in it, 10 times with each load using 5 different mags.

The 6920 (with the grease cleaned out & relubed with Slip 2000 EWL) digested 100 each of first Win Q3131A & then the Mk 318, without any trouble at all. Part of that was doing the lock back test with a mag with only one round in it, 10 times with each load using 5 different mags.

Then I did the lock back test 10 times on the PPU, no problems.

Then I shot off a 30 round mag of the PPU, again no problems.

Then I put in a second 30 round mag of the PPU. Bang-click... short-stroked. Cycle the action to chamber a round manually...bang-click...short-stroked. Repeat one or two more times, same result.

To avoid breaking open another sealed 200 round pack of the PPU, I then took rounds out of the mag to put one each in the other mags I had with me to run the single shot lock back test. 2 or three times in a row it failed to lock back. Then two or three times in a row it locked back. Then I reinserted the mag I had in when this started, which at that point had 20ish rounds still in it. It functioned properly, down to lock back on empty.

At that point, with about 500 rounds fired through the two carbines in about 3 hours, I called it a day.

So, what happened with that run of the PPU not working right, and can I assume it's just the ammo and I can rely on the carbine if I load it with Mk 318?

[bluejackets92fs]

Sounds like the ammo is the problem. I've never had issues with PPU ammo. I'd shoot the rest in another carbine or just sell it and buy Federal instead.

[Barnacle Bill]

Sounds like the ammo is the problem. I've never had issues with PPU ammo. I'd shoot the rest in another carbine or just sell it and buy Federal instead.

I haven't had much trouble before this. I had a CMMG that didn't like PPU 75gr OTM, although it ran fine in other AR's. All this shooting of it has put a pretty big dent in my PPU M855 stash, with only one or two 200-rnd packs left now. I think I'll replace it with Federal as you suggest, since that is starting to show up online at reasonable bulk prices. I have a whole bunch of PPU M193, but I haven't seen any issue with that (in all honesty I haven't shot a lot of it yet, though). If I was going to use it for anything besides practice, I wouldn't want M855 but my understanding is that Mk 318 is more or less the same trajectory and the 9620 now has an ACOG on it with an M855 ballistic reticle.

So, next back to the original issue in this thread, the LW carbine build and its stovepipes. I have a BCM bolt rebuild kit on hand I can install, but since it worked fine with the Mk 318 and only started acting up when I switched to the PPU M855, maybe the thing to do is just shoot a bunch of other ammo and see what happens. Interestingly, the 6520 had been zeroed at 50Y with HSM's 75gr TAP-clone load, and the irins only need one click to the right from that zero to be on with the Mk 318. I have a lot of that stuff, so maybe I'll see how it does in the LW. Or, I can drop the bolt from the 6520 into the LW and see if that fixes the stovepipes (the 6520 didn't have any problems through 100 rounds of the PPU M855).

[bluejackets92fs]

Just for shits and giggles. Check your rings on your BCG and make sure the slots haven't lined up.

[steyrman13]

I haven't had much trouble before this. I had a CMMG that didn't like PPU 75gr OTM, although it ran fine in other AR's. All this shooting of it has put a pretty big dent in my PPU M855 stash, with only one or two 200-rnd packs left now. I think I'll replace it with Federal as you suggest, since that is starting to show up online at reasonable bulk prices. I have a whole bunch of PPU M193, but I haven't seen any issue with that (in all honesty I haven't shot a lot of it yet, though). If I was going to use it for anything besides practice, I wouldn't want M855 but my understanding is that Mk 318 is more or less the same trajectory and the 9620 now has an ACOG on it with an M855 ballistic reticle.

So, next back to the original issue in this thread, the LW carbine build and its stovepipes. I have a BCM bolt rebuild kit on hand I can install, but since it worked fine with the Mk 318 and only started acting up when I switched to the PPU M855, maybe the thing to do is just shoot a bunch of other ammo and see what happens. Interestingly, the 6520 had been zeroed at 50Y with HSM's 75gr TAP-clone load, and the irins only need one click to the right from that zero to be on with the Mk 318. I have a lot of that stuff, so maybe I'll see how it does in the LW. Or, I can drop the bolt from the 6520 into the LW and see if that fixes the stovepipes (the 6520 didn't have any problems through 100 rounds of the PPU M855).


I assume the 6520 is your 6920, just a typo and not a third weapon.
Still only malfunctions are with the PPU....I think you can assume it was ammo related and some lube by using the grease instead of oil. I would confirm in your LW build with other ammo though.

You could also call PPU with the Lot numbers to see if they have had issues with that lot. Are all the Battle packs you have from the same Lot that you have been trying with errors?

[Iraqgunz]

That is a wives tale and has ZERO impact on the operation.


Just for shits and giggles. Check your rings on your BCG and make sure the slots haven't lined up.

[Barnacle Bill]

I assume the 6520 is your 6920, just a typo and not a third weapon.

No, the 6520 is indeed a third weapon, a Colt carbine model that comes from the factory with an A2-type carry handle upper receiver & 16" pencil barrel. It was brought into this matter in response to Iraqgunz' suggestion "Use an AR that hasn't been dicked with at ALL and then fire the suspect ammo. If it malfunctions then you can start to blame the ammo. I would try the lock back test first for about 10 rounds."

The result of that test was it passed the lock back test 10 out of 10 times each with both Mk 318 & PPU M855, and digested a total of 100 rounds of each without incident. It did so from a state of being freshly cleaned & lubed with Slip 2000 EWL instead of the incumbent Tetra grease, as suggested by several including yourself.

The 6920 got into this because of a suggestion upthread to swap bolts in the LW with one that was known to work. In attempting to confirm that it was still working after its recent trip to ADCO, I ran into the short-stroke problem.


Still only malfunctions are with the PPU....I think you can assume it was ammo related and some lube by using the grease instead of oil. I would confirm in your LW build with other ammo though.

Well, the short-stroking on the 6920 actually started on the Mk 318, after having already digested without issue in the same range session 90 rounds of it plus less than 10 rounds of 1980's-vintage Norinco. It then continued short-stroking with the PPU, almost every round. That was with well-aged grease on the bolt & carrier.

Coming back to the 6920 after cleaning & relubing with Slip 2000 EWL, then it digested 100 rounds each of Win Q3131A & Mk 318 plus 30 rounds of the PPU. At that point, it started short-stroking again for 2-3 rounds, then failed 2-3 single round in the mag lock back tests, then miraculously cleared up & started passing lock back tests and finished up the rest of the mag the problem started with without further incident.

So, at this point I think the 6920 looks fairly GTG, although I'll probably get some Federal or PMC X-TAC M855 and see if that works OK.

I agree that next is to try the LW build, cleaned & relubed with Slip, with some different ammo. Debating whether to install the BCM bolt rebuild kit, or at least the extractor spring & insert (and maybe o-ring) from the kit. Also to swap in the bolt from one of the other carbines if it acts up with anything besides the PPU M855.


You could also call PPU with the Lot numbers to see if they have had issues with that lot. Are all the Battle packs you have from the same Lot that you have been trying with errors?

Unfortunately, I didn't save the boxes from the one I was shooting, and I shot it all up. The ones I have left are different lots from each other, so probably from the one I was shooting as well. IIRC, I bought this stuff one pack a month back around 2008, kind of slowly building my stash.

[steyrman13]

Gotcha, I guess I got lost or confused this thread with another one that popped up the same time with similar issues and lost track of the 6520 coming into play.

It might be a combination of lube and ammo together with it sitting right on the threshold of working/malfunctioning. Do you have any weak ammo like pmc bronze or the like to try in it?

I was bad about throwing the boxes away before too. I started tearing the lot number off one box and tossing it in the ammo can with the ammo so I would have it available.

[Barnacle Bill]

Gotcha, I guess I got lost or confused this thread with another one that popped up the same time with similar issues and lost track of the 6520 coming into play.

It might be a combination of lube and ammo together with it sitting right on the threshold of working/malfunctioning. Do you have any weak ammo like pmc bronze or the like to try in it?

I was bad about throwing the boxes away before too. I started tearing the lot number off one box and tossing it in the ammo can with the ammo so I would have it available.

Everything I have in hand right now is 5.56-pressure except for some PPU 75gr OTM. The old Norinco stuff was .223-pressure, but I used up the last of it during this saga.

I guess I need to start keeping some kinda record on the lot numbers. I keep a notebook in my range bag & record various things including weapon, load & quantity fired. Just need to add the lot from the boxes along with the load.

[Barnacle Bill]

OK, so I finally got out to the range again with the LW carbine - freshly cleaned & lubed with Slip 2000 EWL - and 10 Pmags pre-loaded from my newly arrived case of PMC X-Tac green tip.

Flawless.

Also the Slip seems to have not cooked off to any great degree, since everything still looks wet.

Finally, although the X-Tac has green paint on the bullets & red primer seal, there isn't green & red confetti all over the action like with the PPU (even after 300 rounds).

So, looks like the LW carbine is now also GTG.

Thanks to everybody who contributed to this outcome :D

[Tzintzuntzan]

Good to hear. A malfunctioning carbine isn't much good for much of anything.

[Barnacle Bill]

Good to hear. A malfunctioning carbine isn't much good for much of anything.

Yeh, and this one was intended for home defense, plus to train my petite little wife.

Forgot to mention above, I did install the BCM bolt upgrade kit in its entirety.

[Barnacle Bill]

OK, so I got the chance to test the PMC X-Tac green tip in the 6920. 300 rounds through 10 Pmags, no problems. I did not clean it after the range session described in my 08-22-13 post, but did take the BCG out and spray it with some more of the Slip 2000 EWL, and squirt some into the holes in the side (the ones visible through the ejection port).

So, at this point I'm concluding that the issue was mostly the PPU green tip and a little bit excessively-aged Tetra grease.

I'll probably stick with the Slip 2000 EWL now and drop the Tetra. When I bought it, I also bought some of their EWG grease and may test that in autopistols. I bought some Slip 2000 EWL 30 as well, but I think I got a bad or old bottle (didn't buy direct from Slip 2000). Slip says "It contains a proprietary thickening agent that makes this lubricant consitency like a 30wt. motor oil. It pours almost like honey." My bottle was more like old honey after it sets up in the jar and won't flow even if you hold the jar upside down, but you can still dig it out and spread it on something. I am interested in a product that will neither migrate away from where I put it on the weapon or set up if the weapon sits in the safe a long time, so I may order a fresh bottle of the 30 direct from Slip.