AR15.Com Archives
 DMPS LR308 extraction issues
limaxray  [Team Member]
11/24/2011 1:17:57 PM
Just put together an all-DPMS LR308. All parts are factory. Barrel is not new but has < 100 rds through it. All other internals (BCG, LPK, etc) are all new.

Barrel is marked 7.62x51, not .308.

Bolt closes on a .Forster's .308 GO guage, doesn't close on a Forster's NO-GO guage.

Here's the issue––when I chamber a round, it sticks, badly. There is simply not enough energy in the firing sequence to move the bolt back enough to extract the empty case. So I get a shot, then have to drop the rifle butt on a table while pulling the charging handle to extract the case. Feeding is just fine.

It's so bad I get the same result on an empty, resized case. Won't come out without a little percussive maintenance.

Any ideas?
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Dano523  [Moderator]
11/24/2011 3:19:11 PM
Run your ammo (and sized case) through an ammo test gauge to check for reload ammo problems.

Could be a problem with not correctly case trimming, slightly buckling the case shoulders during bullet sizing, not fully resizing the case correctly, or OAL longer than the barrel leade.


Does this same problem happen with brand new factory ammo,
limaxray  [Team Member]
11/24/2011 7:18:40 PM
All my empties checked good on a Forster's .308 case guage. A live round of S&B 7.62x51 gives the same results.
Dano523  [Moderator]
11/24/2011 7:49:47 PM
Scrub the chamber with a chamber brush, then the upper off the lower, the B/C and charging handle removed from the upper, and the muzzle straight down, drop a live factory loaded round into the chamber, give the back of the round a love tap with a wooden dowel to make sure it fully seated in the chamber, then turn the muzzle up, and the live round should fall back out.

If the factory round does not drop out, you have a problem with either the chamber not reamed correctly, or the barrel throat leade too short (bullet tip embedding at loading,which you do not what).

If the round falls out, and the head space checks out, go to the next step,

Now with the carrier alone(no bolt),drop it in the upper, push it up against the barrel extension, then turn the muzzle up again, and the carrier should free fall out of the upper. If it does not, confirm that the gas tube is correctly aligned with the carrier key, and the key is not too wide and/or binding up in the upper slot.


Next, via a piece of tubing, blow down the gas tube from the inside of the receiver, and confirm that you get good air flow to the barrel bore via the barrel gas port index to the gas block passage. If you don't get a good flow down the tube, then the gas block needs to be pulled to check the alignment, and even the gas tube to block.
Note, if the gas block has to be corrected, then don't forget to recheck the gas tube to key alignment again (and tweak the tube if needed as well).
limaxray  [Team Member]
11/25/2011 10:06:49 AM
Test 1: Chambered round falls out of upper without any resistance. No marks of any kind on the bullet to indicate impression into the rifling.

Test 2: Carrier comes of out upper without any resistance.

Test 3: Gas tube is clear and aligned correctly. (Plus, I don't think this is the issue because the FTE occurs when manually cycling a round in the chamber, not just when firing.)

Don't think it's a lube issue, either, because I've been shooting AR15s for about 20 years now and have never seen this issue with any of my others. Granted, there may be some differences because it's an AR10, not an AR15, but I doubt they are significant except for scale.

Really appreciate the help, this one's stumping me.
Dano523  [Moderator]
11/25/2011 12:05:51 PM
Almost sounds like the free space between the bolt lugs to barrel extension is wrong.

You stated that you are getting the correct chamber head space (recess in the bolt, back of the bolt lugs to barrel extension lugs, to chamber shoulders are correct, so best guess is that the front of the bolt from the recess forwards is what is too long.


Double check the bolt face front, and even the barrel breach face to see if that is where all the binding is occurring that is locking up the action beyond normal, or if the problem in on the other end, with the bolt bugle section not correctly milled, and the tail of the bolt binding upon the carrier channel for it.


As for the gas system, do check to confirm that the key in not leaking from the carrier as well.

P.S,. you have CLP lubed the ever loving piss out of the upper bearing surfaces for rifle break in, correct????
ardeambuild  [Member]
11/28/2011 3:45:05 PM
Hello, I am new here but a Old time reader and I enjoy the excellent knowledge here and a NRA Member, I am having the same problem with my 308 20" DPMS Upper sort of like you have? I can cycle 10 rounds of Soft point ammo made by Win but the privi match ammo will cycle one round then not full forward the second round and I have to use a stick to open the bolt along with the pulling on the opp handle, it will not forward either to fire the partially chambered round. It feeds US Match ammo $$$ I consider that a picky cat of a gun, my SA M14 will eat any 308 in good shape with no issues. I literally only have 25 rounds through it? I cant figure it out? I push in a round and it falls out? It seems to cycle even empty tight is that normal?
I like spell checker here, I need improvement. Thanks

limaxray  [Team Member]
11/28/2011 9:29:11 PM
So I tore apart the BCG to look for signs of wear. There's a little on the bolt boattail, but that looks more like standard finish wear as it gets rotated in the bolt carrier.

The two interesting things are:



It makes sense that as the bolt closes, you would see wear only in the last little bit of travel as it locks into the breech face. But I think it's interesting I'm only seeing wear on essentially 1/3 of the bolt face.

Second, and much more dramatic:



There's a shiny spot at the back end of the lug on the extractor (both sides). Looks like there might be a burr or two in the barrel extension. Not sure why it would only be a real player when there's a cartridge in the chamber, though.

Am I diagnosing this right? Are there other areas of the bolt/BCG I should be looking at?

If it is a burr, how do I get it out of there? Take the barrel extension off the barrel?
Dano523  [Moderator]
11/29/2011 9:08:38 PM
No, you have found the problem,

With a round loaded on the bolt, the barrel extension lugs are bind up on the extractor.

Could be a simple as the rim relief channel on the extractor having burs and not allowing the extractor to center set far enough to clear the lugs, but get DPMS on the phone to go over the problem since it could be either the extractor or barrel extension that is out of spec and needs to be replaced.

ardreambuild  [Member]
12/3/2011 10:07:18 AM
Thanks for the insight on the bolt, I fixed the problem last night by lightly filing the extractor areas down until the ammo would flow through with no more Jamming rounds. I used a small triange file that did the trick, I removed only as much as I needed to to cycle ammo through my mag. It Worked excellent and I want to say thanks you gave me hope in humanity!. Used a 22 in my youth then the gun grabbers moved in, I will never give up my Guns. So there gun grabbers!
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