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Old November 20th, 2017, 03:10 PM #1
daNattyFatty daNattyFatty is offline
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Help me diagnose stoppages in my SBR

This morning at the range, my 11.5" ar15 experienced 6, what I believe to be, failures to extract. The fired casing was either still completely inside the chamber, or like a 1/2 inch extracted, with the next round attempting to be fed into the chamber. I cleared each one easily, by dropping the mag and running the charging handle. This stoppage happened on each of the four mags that I used. Here are the pertinents:

Upper was bought as a complete BCM 11.5" 5.56.
Bolt Carrier is BCM, full auto.
Bolt is BCM mpi and hp. I have not changed any o-rings or springs in the extractor.
Buffer is a Vltor A5-4
Buffer spring is Sprinco green
Lower is a Stag.
Ammo is IMI Independence 5.56
Mags are USGI 30 rd, with magpul anti-tilt followers.
This rifle was brought to one of Chad's armorer classes and was "within tolerances" I guess is the way to put it.

A month ago, I used these exact same mags and ammo in a bone-stock M&P15 and did not experience any stoppages. For various reasons, this is only the second time that I've taken this SBR to the range. The first time was almost 2 years ago. I don't remember any stoppages or which mags I used (lesson learned for not taking notes). But I can guarantee that the ammo was M193.

I did not think to examine the case rims from today for any damage. I also did not perform any diagnostics while at the range, just cleared the stoppage and kept firing. I do plan on going back to the range with this particular rifle, hopefully next week, where I can test different mags and ammo through this rifle, as well as any tests that you all can suggest.

So, with all that said, can anyone point me in possible directions, please?
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Old November 20th, 2017, 03:25 PM #2
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I'm not an armorer, but it sounds to me like the extractor isn't grabbing the case or is losing the case as the bolt cycles. I'm sure those with more diagnostic skills will be along shortly with good advice...but if you have the casings can you snap and post a pic of the round with a focus on the case rim?
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Old November 20th, 2017, 03:27 PM #3
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You may have a significantly overgassed system. What gas length are you running? Particularly if it's a pistol length system, you may need to increase the dwell time so that the case has the opportunity to contract after firing and not "stick" in the chamber. The overall force of the bolt starting back may be too high due to this and strip the extractor off of the case. It sounds like you can easily extract after the shot is fired so this sounds very plausible. Do you see a difference in the number of failures when using different ammo, specifically different bullet weights?

A couple of things to look for...

Is your ejection pattern forward of the gun? If so, it's slightly or severely over gassed. Are the cases that properly ejected damaged, dented or flattened? This may be another sign of overgassing. You may also notice an increased recoil sensation because the BCG is slamming into the end of the buffer tube which could show up as a flattening of the rubber/plastic tip of your buffer. Look at the extracted cases and see if they are badly scratched or damaged on the extractor lip as this would mean that too much force is needed to pull the case out.

First thing to try would be a heavier buffer to increase the dwell. A stiffer buffer spring may also help. Second would be to install an adjustable gas block. I had some significant overgassing issues with a SBR that persisted through increasing the buffer weight to 9oz which is very high. The only thing that solved my problem was to put in an adjustable gas block and now it runs like a swiss clock.

As mentioned above, if these things aren't present, it may just be a bad extractor or extractor spring. Try borrowing a BCG from someone else and see if the problem continues.

I doubt this problem is magazine related.
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Old November 20th, 2017, 03:30 PM #4
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Help me diagnose stoppages in my SBR

Was thinking the same thing, but also sounds like your buffer might be too heavy not allowing a full cycle or conversely being over gassed and cycling too quickly.

I donít have any experience with the vltor extension system, but have had a similar setup run great with a standard h3 buffer and carbine spring.

Iíd start with the buffer/ spring and see if making it lighter/ heavier works after checking your extractor spring to make sure it is properly functioning.
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Old November 20th, 2017, 03:39 PM #5
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Quote:
Originally Posted by Jollyllama View Post
Was thinking the same thing, but also sounds like your buffer might be too heavy not allowing a full cycle or conversely being over gassed and cycling too quickly.
It seems unlikely that the buffer would be too heavy as the OP mentioned that the next round from the magazine is jamming into the unextracted round. If the gun were short stroking due to a heavy buffer, it wouldn't pick up the next round from the mag. As I mentioned above, I believe your second scenario of over gassing to be more likely.
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Old November 20th, 2017, 03:40 PM #6
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In my experience, 98% of extraction problems in SBR's under 12.5" lie in the buffer/spring combo. Try a T2 buffer and a standard carbine buffer spring.

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Old November 20th, 2017, 03:44 PM #7
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Quote:
Originally Posted by DubTap21 View Post
In my experience, 98% of extraction problems in SBR's under 12.5" lie in the buffer/spring combo. Try a T2 buffer and a standard carbine buffer spring.

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Unless he swaps out the vltor system he canít as they arenít compatible from my understanding.

First stop should be the extractor spring. It worked before, maybe your extractor spring broke and that is all it needs.
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Old November 20th, 2017, 03:50 PM #8
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Thank you all for your quick replies and suggestions. I do have spare buffers and springs laying around, but it's my understanding as well that they aren't compatible with the vltor system. I'll look into the extractor spring first. My vise isn't set up yet, so I'd have some difficulty un-staking my castle nut to remove the vltor system and get a new stock and receiver extension on.

I did not notice the extraction pattern, nor did I get any pics or observe spent casings.
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Old November 20th, 2017, 03:52 PM #9
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I too think it is overgassed. I would suggest putting in an adjustable gas block rather than trying to dick around with a proprietary buffer/spring combo.
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Old November 20th, 2017, 04:14 PM #10
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Lube ? its been sitting for two years ?
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