A carbine gas 14.5" would benefit from an H2 buffer in my opinion. Especially with 5.56 pressure ammo.
What extractor spring? How many rounds on it? Make sure it's the latest Colt carbine spring, if it's a rifle spring add an O-ring.
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A carbine gas 14.5" would benefit from an H2 buffer in my opinion. Especially with 5.56 pressure ammo.
What extractor spring? How many rounds on it? Make sure it's the latest Colt carbine spring, if it's a rifle spring add an O-ring.
Someone at work had intermittent issues like this. I inspected the bolt and the ejector was dragging in the bolt. I removed the ejector and found a small metal chip in the ejector hole. I completely disassembled the bolt, cleaned and lubed all the parts and reassembled the bolt. That was all I needed to do to correct the issue.
The BCG came complete from Specialized Armament, advertised as "BOLT GROUP, LE/MILITARY, COLT FACTORY TEST FIRED". Bolt carrier is stamped "C" on the left side. Key appears properly staked. Gas rings look new & shiny.
Extractor spring has the black insert, no o-ring. Some of that red debris was even inside the extractor channel in the bolt.
Other than the insert color, I couldn't tell a carbine extractor spring from a rifle extractor spring. I've been thinking about getting the BCM bolt rebuild kit that I think is based on the USGI Crane reliability kit (HD extractor spring, new black insert, Crane o-ring, new set of standard gas rings just in case). However, see my next post.
Last edited by Barnacle Bill; 08-18-13 at 14:00.
Well, this has been my unlucky weekend for AR's. I have this Colt 6920 that I bought NIB about 4 years ago and until this month left it bone stock - whatever it came with from the Colt factory except an assortment of slings (on the factory swivels) and BUIS & optics (on the factory upper's top rail). Until I messed with it this month, it was always 100% reliable with a wide assortment of ammo, including on one occasion the year I got it of over 800 rounds of Win Q3131a without cleaning or relubing (started out lubed with Tetra grease) over the course of a 3-day carbine class.
So, I figured I'd swap the bolt from old faithful into the new LW build, as discussed above. However (a) I'd never fired any 62gr in it before, only 55gr & 75-77gr, and (b) I decided to convert it to a SOPMOD block IIish build so I sent it to ADCO to get...
...barrel shortened to 14.5", threaded, M4 profile and BCM A2X permed
...DD RIS II FSB installed as he put it back together
So, I figured I would check out the newly-modified 6920 and in the process zero it with the Mk 318 before doing any bolt swap testing. Unfortunately that didn't work out so well. The first 5 rounds were the last of this old Norinco 55gr stuff left over from c. 1990, to make sure I was on paper. Then 100 rounds of Mk 318. I was also checking out 4 new PMAG Gen3's with the Mk 318, so I was loading one round from another mag then installing a full 30 round mag and firing with that (off & on while zeroing BUIS & and ACOG) until the bolt locked back on an empty mag. Then repeat with another mag. Three mags - no problem. By that point I'm into the last 10 of the 100 rounds of Mk 318 and it looks good. On the 4th, with the last of the Mk 318 on top (plus one in the chamber) and the rest of the 30 rounds in the mag filled up with the PPU, it short strokes. I repeated this 3 or 4 times, and initially blamed the mag. Then I switched back to mag #3 that had passed the test a few minutes ago - still short stroking. Then I got down into the mag and in fact fired 30 rounds total of the PPU, with intermittent short-strokes all the way down the mag.
By short stroke, I mean the bolt is heading for closed without picking up the top round in the mag. I was definitely checking that the mag was locked in. Sometimes when this happened the bolt wouldn't even reach lock-up, apparently hanging up from the friction against the top round in the mag.
So, now I have to fix this problem before I can go back to working the LW build. I think that's the right order, because the 9620 worked perfectly until I messed with it (not that I can really go back short of a new barrel). So, I'm going to thoroughly clean & lube it, then go back to ammo I know functioned perfectly in it before (my remaining stash of Q3131a and the now readily-available XM193) and take it from there.
It's enough to make me snatch myself bald![]()
So the only thing causing issues in both rifles is the PPU 62gr? Sounds like there's your problem....
I doubt cleaning it is going to do anything for your situation.
1. Use an AR that hasn't been dicked with at ALL and then fire the suspect ammo. If it malfunctions then you can start to blame the ammo. I would try the lock back test first for about 10 rounds.
2. If it functions then I would go back and look at the other AR. Since we know work was done you need to check the gas tube, FSB and make sure that the carrier key is torqued and staked.
3. I would stop using grease. JM2CW.
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I would do what IG says first.
Is it possible that you lost count and the last 2 rounds of Mk 318 were in fact the PPU as well?
The PPU is definitely giving you issues on both weapons for sure. The MK318 made it 98 before failure or possibly miss counted 2 rnds and no failures.
Also as IG said use quality lube not grease
I tend to agree, except maybe eliminate anything implicating the ejector per T2C's suggestion. The 6920 has thousands of rounds through it and while I routinely remove the extractor for cleaning I've never had the ejector out. Crud in there wouldn't cause the short-stroking, at least I can't think of how it could, but it could relate to the stovepipe issue on the new LW build. Anyway, on the 6920, going back to ammo it used to work with seems a reasonable step, and starting that test freshly cleaned & lubed just eliminates one more variable. Now that I think about it, my first AR build c. 1990 initially had some sort of problem (details of the symptoms escape me after nearly 25 years) that turned out to be as simple to fix as running a pipe cleaner through the gas key. Clean couldn't hurt.
Fortunately, this I can do. I have a Colt 6520 that hasn't had any work done on it except by Colt (out of the box the feed ramps in the upper were not aligned with those in the barrel extension, they fixed it under warranty). It doesn't have anywhere near the round count of the 6920, but never had any issues feeding & ejecting.
The LW upper from the OP was assembled by a local smith who builds a lot of AR's and has a lot of repeat customers. He had to mount the FSB onto the barrel and install the gas tube. All those parts were new. It is possible something is wrong, but I can't find anything loose or obviously misaligned. As the barrel is unmodified from the way it was made, I'm assuming the gas port is the right size for that length. The BCG was a new Colt, and all the local smith did with it was drop it in and test fire it. Gas key appears properly staked and I can't detect any looseness.
On the 6920, ADCO would have had to remove the FSB & gas tube to install the rail system. Again it is possible something is wrong, but I can't find anything loose or obviously misaligned. It could be the gas port is smaller than ideal for a 14.5" barrel, since it was made as a 16", but I would have expected a highly experienced shop like ADCO to raise that issue if it was commonly a problem (I get the impression they shorten a lot of barrels). The factory BCG is stock,
gas key appears properly staked and I can't detect any looseness.
So, if the 6520 runs the new ammo like a champ, I'm probably going to need professional help to sort out the two bad boys.
Care to elaborate? As I say, I've had great luck with it over the years. Its pretty hard to imagine a LW grease would cause the 6920 to start short-stroking after 100 rounds as a 14.5" when the same gun ran over 800 rounds on one application of the same grease as a 16". However, I'm open to being learned better.
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