AR Malfunction - Please help diagnose

The rifle: BCM mid-length upper w/ 16" barrel, full-auto BCG (BCM), Carbine Buffer (3 oz), unknown buffer spring composition. I don't have a solid round-count on the upper, but it's likely in the 1000-2000 round range. Up until the problem it's experiencing now, it has run flawlessly with XM193 and M855.

Condition: Cleaned and inspected after every range trip, breakfree used to clean/lube, along with a light application of remoil on the portions of the bolt assembly. No lube or cleaning of the gas tube.

The magazines: 20-round brownells aluminum mags with stock followers, and 30-round brownells aluminum mags with magpul followers.

The ammo: American Eagle XM-193, approximately 8 years old.

The problem: After approximately 60 good rounds down the pipe this weekend, it began intermittently (randomly) wedging a round between the bolt and the chamber. I'm including a picture of the problem I found on the internet, which depicts my problem, but note the pic is NOT my rifle (no way I'd grease the bolt like that). There was no problem ejecting the round before each jam, though I did not have the opportunity to view ejection to see if the spent casings were being thrown with the same force/direction as when a jam did not occur. Upon clearing the jam, most of the jammed shells had a pretty good dent in the case from where the bolt contacted the case body.

I know this problem occurred with the 20-round mags. Unfortunately, I don't know for sure that it did with the 30-round mags, though if memory serves it occurred with both.

So, Highroad Oracle... whatcha got for me? I have a couple ideas of what might be happening, but I need some help with this one.

badkarmamib said:

Looks like it is short-stroking. The bolt is not coming back far enough to get completely behind the next round. Possible causes are gas leaks, or dirty components.

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Definitely not dirty... like I said I clean well after every range trip. I've inspected the bolt, gas rings, and gas tube (as far as possible without removing it) and see no issues. And the problem is intermittent, with the majority of rounds working fine. I'd say by the end of the day one out of every 5-10 rounds would malfunction. Could be just coincidence but it seemed to get worse as things went along so I'm wondering if heat is playing a part in the issue.

jhb said:

not really gonna help, but for my own info to learn from your problem.......... all mags had been used before with no issues until this last time? springs are still good in the mags? thank you Sir.

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Great question. Unfortunately I can't say for sure. The same type of mags have been used before with no issue, but not necessarily these exact same mags. What I do know is that the malfunction occurred with at least 2 different 20-rounders, and probably 1 ten-rounder, so while I can't say for sure, I'm leaning towards this NOT being a magazine issue. Springs are like new as are followers.

243winxb said:

Number the mags to see if one or all do it. Lube the magazines. If rounds are slow moving up in the mag, the bolt will go over top.

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I appreciate the advice. This was one of my thoughts, but I've inspected all the mags I had with me this day, and they're all clean as a whistle and very lightly lubed. Testing them manually shows zero hesitation to push the next round up as expected. Springs are like new and have had very few (if any) cycles on them.

The only time I use 20-round mags is when I'm sighting in, and that's once in a blue moon. These mags were either brand new, or at most had been loaded once or twice.

badkarmamib said:

I have seen new magazines with pretty good burrs on the bodies and followers. Brownells are supposed to be some of the best, but that would be relatively easy to check. Use a ruler to depress the follower repeatedly, and feel for dragging. Push down on different areas of the follower while doing this. Is your gas key still secure? It sounds like you have quality components, but it is still best to check the easy, common culprits first.

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Already done everything you mentioned. I've disassembled the mags, checked for burrs, manually tried to get the follower to hang up by pressing on different areas. I can't get them to do anything but work the way they're supposed to. Even when I push the follower down at ridiculous angles that would not ever be seen in actual operation they pop right back up perfectly. No dirt or debris either... clean as a whistle.

Check on the gas key - still secure & well-staked. Gas rings show no visible damage or wear.

badkarmamib said:

How does your buffer, spring, and tube look? Any wear marks? And scuffs on the BCG? It just sounds like something is slowing the BCG down.

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The buffer spring looks fine... I don't have a way to measure the spring rate, but I compared the length to a brand-new spring and it definitely hasn't shortened (same length). The inside of the buffer tube looks smooth as a babies butt, and is very lightly lubed.

BCG looks fine as well. Nothing out of the ordinary.

Is there any possibility the bolt is cycling too fast for a (properly functioning) magazine spring to keep up with? If so, could a weak buffer spring tube possibly cause such an issue? Not saying it is weak, but it's the only thing I can think of in this assembly that's been cycled enough to possibly cause a degradation in performance.

Varminterror said:

Is it short stroking, or not? Is it a common mag, or common position in the mag? Is your gas block tight and free flowing? Are your lower bolt lugs in good condition? Is the bolt bouncing? Or is your bolt travel too long?

Answer those questions and you'll find the problem. It's an AR, a remarkably simple mechanism. It'll tell you what's wrong if you ask.

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Gee, thanks so much for the help. I just tried asking it really nice to tell me what its problem was, but apparently it has yet to master human speech.

Is it short-stroking? No high-speed camera here so I don't know. I'm sure that could be my issue, but it seems my issue could also be that the bolt is (for any number of reasons) already over the top of the next round on its return-trip home. Is that what bolt-bounce is?

Yes, the lugs are in like-new condition, as they should be for a 1-2K round rifle. I don't know what you mean by bolt-travel being too long.

But I don't know why I'm telling you this. I should apparently be whispering it lovingly to the AR and waiting for a response. You'll have to excuse me. I'm apparently also a very simple mechanism.

mjsdwash said:

Looks very much like slight short stroke where the bolt lugs stop on top of the rim, pushing it down, and push the case in the extractor groove. May be a weak gas pressure, and the ammo your using is lower than before. I have seen one rifle do something similar, where after 60 rounds it would short stroke just enough to feed, but not lock the bolt back. I replaced the gas tube, carrier key, and gas rings, and the problem remained. Replacing the gas block fixed it, and SEVERELY increased recoil. The recoil was not bad but it was extremely light before. A bad gas block is likely to be problematic from the start, but the tube, rings or key can go. Some say if you shoot 22lr through one it can block the tube, though Ive ran 500 or more many times with no issue. A fully gased carbine can easily cycle on gas rings in any position, but you may as well check that out. One less common thing to consider is wear, or damage to the lower bolt lugs, that pick up the cases. If you have a caliper, someone here can compare the dimensions. Hope it helps!

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Much appreciated. The short-stroke you describe is the only way I've been able to manually duplicate the issue... positioning the bolt face in the depression between the rim & case and then letting it go pretty much replicates the problem. But the thing I can't understand is that if this was the case, why would the problem be intermittent, with most of the rounds chambering fine? And I'd think that as a rifle heats up, if anything more gas pressure would develop.

I did check the gas ring gaps... appropriately staggered upon disassembly. I have not yet had a look at the gas block however.

Maybe its time to just start testing things out instead of conjecturing I was hoping for an easy answer lol. They don't come very often.

Varminterror said:

Just have to know the questions to ask. It's obvious you don't.

I wish you luck.

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Once again, thanks so much for the help. If I wanted to deal in riddles I'd have asked a Sphinx. Next time you have no useful input, do everyone a favor & just stay out of the thread.

I may have possibly found the problem, though I'm not sure.

Should there be any leakage at all around the gas tube where it connects to the gas block? Should it be a perfect seal?

The reason I ask... there's some light carbon build-up on the outside of the gas tube extending approximately 1" from where it connects to the gas block. Not alot... I can scrape it off easily with my fingernail... but enough to blacken that 1" section of tube.

ny32182 said:

Does it lock open on empty every time? That may give you a clue about whether it is short stroking vs. bouncing.

It sounds to me like a short stroking/lack of gas issue. It may be possible that your gas block is leaking or misaligned slightly, even if it isn't visually obvious.

If the buffer spring is of unknown history, it wouldn't hurt to drop in a new milspec spring; it would rule it out as an issue and they are cheap.

The only other thing that hasn't been touched on is ammo. Federal XM193 should be good, but nothing is perfect... I would try a different lot of ammo if that has not been done as well.

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Unfortunately it wasn't happening regularly enough in the field to tell. It did lock back on empty mags reliably but since it wasn't consistently malfunctioning that doesn't mean much. When I go back out to try to figure this out I'll try loading just a couple rounds at a time to see. Thanks for the advice

Got new buffer springs, so I'll give that a shot.

I didn't think 8 years was enough to degrade the ammo, but maybe I'm wrong? What I do know is that the rifle has, until now, run that particular lot of ammo without a single hiccup, so I'd think that unless age was the culprit the chances of a previously-good lot of ammo going bad at the rate of 1 in every 5 to 10 rounds would be surprising. FYI the ammo's been stored inside (in original paper packaging) between 60-80 degrees, depending on season, in a very dry climate (except for 2-3 months out of the year when we run a swamp cooler)

ny32182 said:

That is normal.

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That's what I was afraid of