Help diagnose AR feed failure

E

ESQ

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Location
BC
Rifle:
10.5in LMT Defender
Ammo:
75gr Hornady 5.56 TAP OTM
Magazines:
PMag, LAR-10 (happens in all 12 mags tried)

Bolt locks back on single loaded magazines (so it is not short stroking)

Basically, the rifle seems to fail to eject, the bolt picks up a new round and the empty case jams agains the barrel extension lugs to crumple the case mouth with the fresh round piled under it.

I initally was running a full auto LMT carrier with an H buffer, but then switched to a semi auto carrier with the standard buffer. The lighter BCG/Buffer combo improved the situation and the malf became infrequent, but sometimes still occured.

All combinations of buffer and bcg weight were tried, and the lighter, the more reliable. This is irrespective of the bolt used. Both bolts are newish with the crane O-ring and should be very strong extractors.

The malf has never appeared firing full power M855 or M193, just the 75gr 5.56.

Any suggestions? The 75gr runs flawlessly in my 14.5in LMT with any BCG/buffer combo. Just the 75gr in the 10.5 causes this issue.

I have a pic from the range today, but I need to get it from my buddy. It just shows the jammed round angling up.

I really want to run the 75s reliably in the 10.5 as this is my favourite ammo/gun combo:mad:
 
I had the identical problem with my SU-16. It wouldn't eject the spent case and would jam the next round into the back of the empty.
I replaced the extractor with a titanium one, and never had the problem again.
 
sounds like you don't have the proper springs for your extractor. you need to have the regular metal spring, D-ring spring (black) and another inner elastomer spring plug. You could also have a failed ejector or spring or both. hard to tell without physically handling the bolt.
 
I don't really think it is the extractors. This isn't a frankengun. Both bolts that I've tried are new factory LMT bolts with crane O rings. I don't know if I can build a stronger extractor.
 
It's not so much the extractor itself that is the issue. the metal extractor is doing it's job. The big issue is that if the bolt was not built up specifically for a short barreled upper, it may not have the necessary elastomer 'springs' to aid in increasing reliability of extraction. If you know how to take down your extractor, you should be seeing the extractor itself, the metal coil spring, a black elastomer D-ring spring, and a elastomer tower spring.

The only other thing I can think of is that the spring in your ejector is weak or the ejector is out of spec and too short.

Without physically inspecting the gun, it is very difficult to pinpoint the cause of your malfunctions. Hope this helps a little.
 
ESQ said:
I don't really think it is the extractors. This isn't a frankengun. Both bolts that I've tried are new factory LMT bolts with crane O rings. I don't know if I can build a stronger extractor.
Click to expand...

It doesn't matter if it's a "frankengun", I've owned/own some quality firearms and many have had mechanical problems at one time or another - some after a lot of wear, and some right out of the box.
The SU16 uses an AR15 BCG
Diagnose, repair, shoot.
 
I'm running into the same issue with my AR 11.5" I never had this issue when I had my stag upper which was 16" I'm also using the original stag bolt & carrier so I know it worked fine.

what is happening is double feeding, It fires the first round then without the bolt going back far enough to eject the first round then starts feeding the next round. The spent round stays in the bolt face and starts to feed the next round. The bolt goes forwards and the spent round gets jammed into the top of the barrel chamber & the second / new round can't feed into the chamber and also gets damaged trying to go into the chamber.
This is my problem.
 
The 75gr Hornday 5.56 uses an extruded powder - that might affect the port pressure curve such that it affect the timing of a very short carbine. Typical M855 uses ball powder. It is also possible that the hornady casing expands slightly more than other ammo, and your rifle chamber is on the tight side of the tolerance.
 
Brocolt said:
I'm running into the same issue with my AR 11.5" I never had this issue when I had my stag upper which was 16" I'm also using the original stag bolt & carrier so I know it worked fine.

what is happening is double feeding, It fires the first round then without the bolt going back far enough to eject the first round then starts feeding the next round. The spent round stays in the bolt face and starts to feed the next round. The bolt goes forwards and the spent round gets jammed into the top of the barrel chamber & the second / new round can't feed into the chamber and also gets damaged trying to go into the chamber.
This is my problem.
Click to expand...
Your extractor spring sounds like its overcoming the ejector.
 
Why all the focus on the extractor? The cartridge case is coming out of the chamber, and being held on the bolt face throughout its entire stroke. The extractor is fine.

The only time I remember this happening to me there was a brass shaving stuck in the ejector channel, making the motion of the ejector sticky. The case would extract and hang on the bolt all the way back and all the forward again. Disassembly, cleaning and lubign the ejector and spring fixed it.
 
BattleRife said:
Why all the focus on the extractor? The cartridge case is coming out of the chamber, and being held on the bolt face throughout its entire stroke. The extractor is fine.

The only time I remember this happening to me there was a brass shaving stuck in the ejector channel, making the motion of the ejector sticky. The case would extract and hang on the bolt all the way back and all the forward again. Disassembly, cleaning and lubign the ejector and spring fixed it.
Click to expand...

You know, you might be right. I remember a shaving in mine when I removed it, but was so freaked out by the "broken" bullets with powder pouring out, that I just replaced the whole thing.
 
Here is a picture of my malf. I have already cleared the mag and the fresh round that was jammed under the empty case leading into the chamber.

Note that the cases, other than the crushed neck from where it slams into the barrel extension, are in good shape. No shavings or metal slivers.

Sorry to the dial up users....



I still don't think it is short stroking as when I load just one in the mag as it always locks back empty. I have never got this malf to occur on single loaded mags.
But then again, it might be shortstroking as the malf appears less frequently when running my lighter S/A BCG and the carbine buffer vs the F/A and H.


I have cleaned, lubed and reassembled the bolts and still get this. I get this with either of my new LMT bolts.
 
greentips said:
The 75gr Hornday 5.56 uses an extruded powder - that might affect the port pressure curve such that it affect the timing of a very short carbine. Typical M855 uses ball powder. It is also possible that the hornady casing expands slightly more than other ammo, and your rifle chamber is on the tight side of the tolerance.
Click to expand...

This is a good point that touches on what my educated guess is on what your problem can be.
I believe short barrelled DI AR's are much less tolerant of barrel port gas pressure variations than their longer barreled counterparts. In order to compensate for less pressure and short stroking(due to the shorter barrel) many makers make their barrel ports larger. You end up getting too much gas too soon (and I'm not referring to farting:)) The extraction cycle starts before the case has shrunk back enough to clear the chamber of the rifle. This causes resistance of extraction and delayed ejection which leads to your feed problem.
Your upper was probably made and tested with issue ball type ammo in mind. So other brands of ammo can give you problems.
I've seen some aftermarket adjustable gas blocks for sale in an attempt to allow guys more ammo latitude. Polishing the chamber and deepening/tightening the extractor grip have been tried with mixed results in the past--but IMHO you'd be trying to overpower the problem rather than dealing with the cause of the symptom.
I hope this helps you;)
 
Hmmm, so I assume that extruded powders are slower burning due to the lower surface area of the grains, so the pressure spike is delayed vs ball powders and the gun occasionally fails to cycle. Would explain the flawless performance with M193 and M855, but not the 5.56 TAP.

I suppose this is exacerbated by the fact that LMT 10.5s run small gas ports to correct the usually over gassed situation in short barrelled ARs. Guess mine is undergassed with TAP!

As an aside, does Mk 262 use extruded or ball?


Whatever, going to go play with my new SAN! Mmmmm
 
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