Mysterious Malfunction

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creekerdoug
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Mysterious Malfunction

Post by creekerdoug » Sat Jan 22, 2011 8:36 am

I took my SBR out for it's first test drive and sighting in session. A half a dozen rounds later it was sighted in and I was happy. It ran great. I emptied the rest of the magazine just testing for reliability. Mission accopmplished, it ran 100%. I let it cool down a little and then put the suppressor on to see how much point of impact change I'd get. I ran 10 to 15 rounds through the suppressor and it ran 100%.

After things cooled off I shot it again unsuppressed. On the second round it failed to fully extract the spent round while feeding the next live round. I tried three diffrent magazines with the same result.

After a complete cleaning and inspection I decided the generic bolt and carrier were suspect so I replaced them with a Daniel Defense BC group. I also disassemble the entire upper and could not find a problem. It all seemed OK so I reassembled the upper.

Now, just like before it runs 100% with the suppressor attached. Unsuppressed, it does the double feed/not completely extracting thing. I took the heavy buffer out and put a regular carbine buffer in. Didn't change a thing. I tried a different recoil spring and nothing changed. The only thing I haven't taken off is the front sight to check for gas hole alignment. I am now out of ideas. Any suggestions?

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S Lance
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Post by S Lance » Sat Jan 22, 2011 10:12 am

Gunworks might chime in at some point with the answer, but in the meantime, I'd check to be sure I was using the correct black insert and on o-ring under the extractor. One of our other members was having a very similar issue with his new upper last winter which was solved by the use of the o-ring. (It was a new BCM upper that would have intermitent failure to extract/bolt override malfunctions.)

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Hairy78
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Post by Hairy78 » Sat Jan 22, 2011 10:47 am

Creek, it sounds like you are under gassed. The suppressor adds a tremendous amount of back pressure. What size gas hole are you running, and what brand barrel are you using?
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S Lance
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Post by S Lance » Sat Jan 22, 2011 11:36 am

Hairy78 wrote:Creek, it sounds like you are under gassed. The suppressor adds a tremendous amount of back pressure. What size gas hole are you running, and what brand barrel are you using?


That is thought, but I would think that switching from a H buffer to a regular buffer would at least help that. I wonder if the bolt locks back on an empty magazine without the can on?

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Hairy78
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Post by Hairy78 » Sat Jan 22, 2011 11:40 am

S Lance wrote:That is thought, but I would think that switching from a H buffer to a regular buffer would at least help that. I wonder if the bolt locks back on an empty magazine without the can on?
Scott, almost any upper with a suppressor on it, running a non-H carbine buffer should be pounding the crap out of the shooter. I run H3's to keep the recoil down. I know I am over gassed, but I want my guns to be reliable with and with out the cans.

I bet he has a little bit of carbon in his tube or block, impeding the gas flow.
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S Lance
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Post by S Lance » Sat Jan 22, 2011 11:57 am

I completely agree that the can increases the gas pressure a bunch. Interestingly, neither of my sbr's will run a H3 reliably. My backup gun uses a H2 and my primary runs a H. I have several thousand trouble-free rounds through my primary gun both with and without the can. I think the whole thing just shows the variability from gun to gun.

I also agree with your assessment that the rifle should work 100% both with and without the can.

If I was in Creeker's shoes, I'd check the extraction first, then the bolt locking back thingy, then take the gun apart to check for gas issues, mostly because the first two steps are easy, and the third a bit more time consuming. I hope he keeps up updated on the issue.

If Creeker would just bring his rifle to the range with a case of ammo, I'm sure we could get it sorted out for him!:D

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bcp307
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Mysterious malfunction

Post by bcp307 » Sat Jan 22, 2011 12:32 pm

I had the same extraction issue last year with my new BCM upper and bolt carrier. I tried changing the buffers, but it did not help.
After installing an o ring, it now runs 100% and has run 100% for about 2,500 rounds (including one school were we shot 1300 rounds in two days).

I had never had to install an o-ring in any AR (including full auto's) until I built this 11 1/2" SBR BCM., and thanks to Scott's suggestion it took care of it.

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creekerdoug
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Post by creekerdoug » Sat Jan 22, 2011 2:57 pm

I agree it seems undergassed. The reason I changed the bolt carrier group was because both the extractor and ejector seemed weak to me. I don't believe I've ever been able to push the ejector flush with the bolt face with my thumb nail on any other bolt. Like I said; it was a generic (who knows where the hell I got it) unit which is why I changed it out for Daniel Defense stuff.

No matter what configuration I use it will lock back on an empty magazine, both suppressed and unsupressed and with the standard or heavy buffer.

When I say I disassembled the upper I mean I really disassembled the upper. I took the barrel and gas tube (which is visually clear) off and both appear to be fine.

This upper is a 11.5" unit. The barrel is from DSA. It's not free floated. It's pretty simple and ordinary. It has a standard front sight on the barrel and it seems to be tight and not canted. I've built a dozen or so uppers over the years and this is the first one I've had any trouble with. I've got an extractor upgrade o-ring which I'll install but the Daniel Defense stuff seems pretty stout without it.

Anyway, thanks for the advice.

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S Lance
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Post by S Lance » Sat Jan 22, 2011 4:03 pm

Color me dense, but I don't see how the rifle can be much undergassed if it locks the bolt back on an empty magazine. I'm happy to be corrected if someone can explain it in a way that I can understand!:)

The thing is, in my opinion, is that sbr's have a hell of a time with extraction. My knowledge of this is gleened from some first-hand experience but mostly by countless wasted hours on the net. (As I've said previously, this means it comes from the ignorant, paranoid, and delusional.) I believe the extraction issue is actually why the o-ring was introduced to begin with. On both of my sbr's, I've always used both the black inserts and the o-rings. Both of them have so much tension on them, I have to use pliers to insert the extractor pin into the bolt.

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bigdaddy
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Post by bigdaddy » Sun Jan 23, 2011 11:06 am

If it will lock back on an empty mag, it is not undergassed.

I would concentrate on the extractor.
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