I've got a brand new DPMS AP4 308 and it is failing to completely extract 1 out of every 5 shots taken.  The spent casing pulls back from chamber but fails to eject free and clear so when the bolt comes forward and tries to feed another round in it jams because there is still the spent casing inside the receiver.

Anyone have any particular ideas on why this might be happening?  I've taken apart, cleaned, and reassembled twice with the same result.  I'm using commercial Remington UMC mostly.  I used some random milsurp from a guy at the range and it did not decrease nor increase frequency of the problem.  Based on that, I'm tempted to assume it's not ammo related.

I've also checked the extractor and extractor spring and both are unbroken and assembled correctly.

This may be the most useful piece of evidence:  Today it jammed on the last round in the magazine recently and the bolt locked back like it should with the last spent casing wedged inside the receiver.  It actually looked like it was stuck in the extractor and not being released by it when it pulled back.

Any thoughts, advice, or past experience?

Update #1 - Heavy Buffer did not end up improving the condition noticeably.  I'm not saying it won't add reliability, I'm just saying it didn't solve my specific problem on its own.

Update #2 - The O-Ring trick appears to have worked.  I dumped a whole mag and not a single jam.  Further testing still required but there is no way it would have made it through 20 rounds without a jam previously.  I used Buna-Nitrile orings size 006.  They were a really tight fit to get on the spring but after a few minutes of cursing I got one on there.  I tried to put two but the ejector would not reassemble with two.  One appears to be just the right amount of additional spring resistance to get the job done.  That said, I'm a bit boggled that DPMS hasn't developed a heavier ejector spring for their 308 Carbines.  It seems like that would have solved the problem right at the factory.

Thanks for everyone's advice!  

Did you disassemble the bolt?Remove the extractor and check under it for debris or brass.

Yes sir, cleaned it as well.  It appears to be in perfectly fine condition.

The symptom you describe is the extractor releasing the case rim too early. That symptom appears more frequently in carbine length gas systems than rifle length gas systems. One method to band-aid the problem is to install an o-ring around the extractor spring to increase extractor tension. A better fix is to slow down the extraction timing by increasing the weight of the buffer (i.e. get a heavy buffer).  The best solution is to install a barrel with a mid-length or rifle-length gas system. Use of ammo with medium burning rate powder will also help, but its hard to do unless you reload.

Thanks a ton for the info.

I'm googlin' right now trying to find heavy buffers.  I've come up with this so far:

http://www.ar15.com/forums/topic.html?b=7&f=120&t=229073

Am I getting warm or are there better options out there?

I've got the factory collapsable stock on my AP4 FYI

I bought one of Slash's heavy buffers and found it to be excellent quality and he shipped quickly. I believe he also has Armalite AR-10 buffer springs which are more consistent than those sold by DPMS.  I run Armalite buffer springs in my 24" and 16" Fulton/DPMS 308s.

Good stuff!  I got ahold of him and he is going ot get me squared away on both the buffer and the spring.

Thanks again for the help!  I really appreciate it.  I'll post an update when it comes in with my results.

try the o-ring first, I use 2 actually.  Get them at any hardware store, 1/4" OD, 1/8" ID.

I use 2, installed like so:


You will have to compress a little to get the extractor pin back in, but it works great.

And o-rings are like $0.39 each.

Thank you very much for the illustration.  That is very helpful!  I'll start with the upgraded buffer but will consider this as well if I need further improvement.

Polish the chamber too.  Reducing friction anywhere you can on extraction will help.    I had some of the softer surplus brass sticking.  Some DPMS chambers are pretty rough, YMMV.

I have a new DPMS AP4 308. Have shot a few rounds and no problems at all.  The first 6 were Win White Box FMJ, the rest are mild hand loads with Rem 180gr Core-lock @ 2275 v.  Good luck finding the problem, I like mine and have shot some piggies with it.

Make sure the ejector is working and as previously posted, check and/or polish chamber.

The weapon has a .308 Winchester chamber.  Are you using 7.62x51 NATO or .308 Winchester?  My DPMS acted like yours with NATO ammo but was flawless with .308 Winchester.  I traded for an AR10 so that I could use surplus NATO ammo.

Update 2009-03-28

I got the heavy buffer and spring in the mail and installed it and it didn't really seem to affect the extraction issue.  I was still getting a failed extraction followed by a jam every 3 to 5 shots.  I'm going to try the o-ring trick next and see how that goes.

I've used both .308 and 7.62x51 and I get the same results with both so I don't suspect that it is ammo related.

I guess if all else fails I'll have to send it back to DPMS.  

Get a chrome silicon extractor spring from Tactical Springs and forget about o rings and d rings.
REM UMC ammo is junk.
Is the extractor snapping over the rim or riding the rim? are there scratches on the rim where the extractor has slipped off while pulling the case out of the chamber?
Make sure the ejector is not hanging up. If the ejector is operating properly it will keep the neck of the case up against the right side of the receiver until the case flips out.
take a small allen wrench, drill bit what ever and push the ejector down into the bolt past the face and release it, work it back and forth a few times to make sure it is not hanging up on the edge.
Next, it could be that the extractor groove is not wide enough to snap over the rim of the case or maybe not wide enough for every case.

I had a lot of extraction problems with my DPMS 16" when I bought it. Since then, I've added an O ring to the extractor, added tungsten weights from a H3 buffer to my factory buffer, installed one of Slash's  AR10 buffer springs and polished my chamber. The rifle finally works fine now with my loads. Some mil surplus is still iffy.

DPMS lists a unique .308 part number for the extractor spring: 308-BT-08K.  The AR-15 spring is the BT-08.

Tactical Springs lists only AR-15 springs as far as I can see.  Is there one available for the .308 bolt?

After you posted I went to the shop to check, the bolts I received earlier last year had 1 spring that I replaced with a CS spring. The bolts I received in Jan and have not used have 2 springs, one inside the other.
I assume at some point they changed, forgive me for not keeping up with those changes.

How do you know? They have been sold with markings for both.