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Posted: 6/26/2005 11:51:23 AM EDT
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I just got a new LMT M4 14.5" upper. the first 30 rounds went through without an issue, about half way through the 2nd mag, i started having an extraction issue. what is happening, is that the rifle is trying to chamber the next round, but is forgetting to extract the previous round. if i clear out that 2nd round and recock the bolt and then manually pull on the charging handle, the round will extract. i have tried multiple magazines since the issue started, and it now does it after every round fired. let it be known that when i received the upper, it was completly covered in oil, inside and out. I wiped off any exposed parts, and i shot the bolt group with carb cleaner. I then wiped down the moving parts with a thin coating of oil. my thoughts are that there was oil inside the gas tube and from the 45ish rounds i put through it before the problem occured, it was enoguh to clog it. my next planned step is to squirt some carb cleaner into the gas tube, and then blow it out with compressed air. any thoughts on this? |
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My bet is the extractor is slipping off the rim of the fired case. It sounds like the BC has enough gas and is traveling back far enough to grab another round. I would take a close look at the extractor. Consider a heavy duty extractor spring if all appears normal with the extractor assy. also make sure the chamber is clean and free of any burrs that might impede extraction. |
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well, being that it is brand spanking new, i would think that there is something that I should check to be wrong/broken... generally I would wait until a part is worn out before i start upgrading. its brand new, so i see no reason why it should be malfunctioning. while at the range, i did find a small piece of brass stuck in the extractor, but clearing it out didnt solve anything. perhaps i will try to take the BC apart to see if anything else is in there. I am using LC XM193 ammo, made in 04, so it is milspec ammo... but i doubt that could be overpowering the extractor since the BC is a CMT, which i believe is also milspec? |
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Do you want the gun to work, or not? Check the archives for massive amounts of posts on this issue, that many many other people experience. Here's the short answer. Pressures are high in carbines, due to shortened gas system. Bolt opening timing is early, and pressure is high in the case when the bolt tries to unlock and extract the case, Result is case stays in chamber, extractor slips over the rim in the typical circumstances, and fails to extract. The fix is to increase the extractor tension on the case rim. This can be done by using a stronger extractor spring, or a D-Fender polymer extractor D-ring, or a Crane O-ring(which basically imitates a D-Fender). These are not expensive items. In some guns it may also be desirable to use some method to retard the opening timing,such as a heavy buffer, but in a new gun this is usually not needed. I'm sorry that your new gun is not working reliably, but I'm trying to instruct you how to fix the problem inexpensively. Increasing the extractor tension is the standard fix for this very common problem. |
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Home depot, Lowes, Ace hardware (plumbing supplies), #60 Rubber O ring ($0.10) added around the outside of the extractor spring will add tension to the extractor. In time (few hundred rounds), the chamber will polish out threw live fire, and the O ring/added tension will not be needed. It just comes down to allowing the rifle to break in, and the O rings solves the new chamber problem to allow the rifle to break in under it’s own power. Note: Still shaking my head that twl posted the solution twice, and you still don't believe him. Now I know Why Tweak just points some of you guys directly to the FAQ.  |
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its not that I don't believe him, its that the rifle is my first AR-15, and I don't fully understand all the inner-workings of it. the first post sounded to me as a 'you bought cheap parts, go buy upgraded parts', which i didn't want to hear because i would like to see this working asap, and not have to purchase parts online... but his 2nd post was a more technical description of the problem, which makes me understand the situation more, and makes me thankful that such people with technical knowledge are available to talk with. your answer that the problem will go away after the rifle is 'broken in' makes me feel better knowing that the problem is an inherent issue of the design, and that my rifle is most likley not defective. i'm going to try that o-ring from the hardware store and try the rifle again tomorrow |
Actually in his case I'm wondering why his upper doesn't work as it comes. the spring hasn't had time to wear and carbines don't require Defenders and such to function, just to function reliably under the worst of circumstances. That's why this is Troubleshooting, we find out what is wrong then fix it. slayer, are you using an "H" buffer? does your upper use a standard FSB? XM193 has been proving finicky in some guns. Did you dry out the chamber before you shot the rifle? |
Ok Papa Smurf  , since you want to put it that way, here are the greatest hits,Basic list of checks, Pull extractor off of bolt and check, Claw grabbing edge for burs, Claw pin holes and pin edges for burs that may be holding the extractor open, Bolt extractor limiting pad for burs, Bolt extractor slot for burs that may be holding the extractor open, Correct spring installed/used for the extractor, Extractor freely moving in the bolt channel Barrel Chamber reamed without tool marks present. And since the rifle ran for a mag, then chances are that maybe the rifle is chunking brass slivers into the extractor and causing a failure to retain, so check. Barrel extension lugs for burs, Barrel extension feed ramps for burs, Bolt face ejector hole for edges burs, Again, the extractor claw edge for burs, Mag front feed section for burs. Machining burs left in any of these area will scrap the brass, and end up not only in the action, but the extractor as well (read copper fouled out extractor). But as TWL, and I both pointed out, any of these conditions (with the exception of a bad chamber) would self-correct threw live fire wear/mating. Simple put, adding a extractor spring O ring, and remembering to lube the upper bearing surfaces with CLP will allow rifle to deal and correct the problems, instead of these problems worsened by being Plinkereye, to answer your question, the above is just a small portion of what is checked by the smith/builder in regards to a completed factory rifle. Since you built the rifle, did you as the smith check the entire rifle to insure that it was built correctly with in spec parts to insure that it would run from the start, or was the rifle just thrown together like a Lego set (read including trusting the vendor who built the sub assemblies). To take this one step farther, my biggest pet peeve is kit sub assemblies that are pre-built before being sent to the end user/builder. When ordering a kit, I will specify that all the parts are loose. This allows me to check each and every part to make sure that it is within spec, and the assemblies are built correctly from the start. Granted that 95% of the stuff coming out of the kit suppliers are built/produced right, its the other 5% that someone was asleep at the wheel when assembled/checked, and end up here being trouble shot, or sent back to the supplies to have him check his work. So, to answer your question, When factory rifles are built, great attention to detail is maintained to insure that parts are within spec, relatively burless, and assembled in a manner to insure that the rifle will not have to be returned for service from the start (in regards to the big builders, with some smaller builder just not given a dam (give you hint, one started with a H, and another with an AS)). |
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ok, just getting back from the range... when i took the bolt apart yesterday, i didn't see anything weird, but i cleaned and lubed the BC/extractor well, and also put an o-ring on the extractor spring. i couldn't really clean the chamber too good because i dont have a proper chamber brush for this rifle yet (who's bright idea was it to make the chamber so funky!) so i take it to the range and put about 20ish rounds through it without trouble, and then i start having the extraction problem again. i thought it might have just been a one round fluke, so i fired another 5-7 rounds with the same problem. i pop the BC out and notice another sliver of brass on the bolt face. i also noticed some tiny brass flakes on the chamber, but i didn't bring any cleaning tools with me, so i coudlnt really do anything about the chamber. i clear the bolt face and was able to fire another 30 rounds without issue. i used the tip of a bullet to pick the brass sliver off the bolt face and to make sure no brass was stuck under the extractor. i noticed that the extractor claw scratched the metal on the bullet really easily... i've checked a bunch of spent brass, and the rims don't look noticeably damaged, but it does support your theory that my extractor claw is shaving the rims off the cartridges, causing brass to get stuck under the extractor.. hopefully you're all right that it's just a 'break in' issue and will go away after a few hundred rounds. so it appears as though the o-ring is going to be my savior for now... i'll take it to the range again this upcoming weekend, and if it jams again, ill check to see if shaved brass is the issue. tweak, the buffer is 'whatever came with my lower'... i got a pre-ban lower through the EE, and the person sold it with a 4-pos colapsable stock. i'd be willing to bet that its a standard buffer. as far as the FSB goes, i didn't order anything special for it, so it's whatever LMT puts on them, i'd assume its a regular one. as far as 'drying out the chamber', i wiped it down with patches on my finger. since i dont have a chamber brush yet, i could only do so good of a job dano, in regards to your comment about the manufactures checking/testing everything, the upper came through with a tag tied to it from LMT with a checklist of everything from them being tested/checked, as well as test firing the upper. the BC and charging handle is a CMT and was 'installed' by the vendor that sold me the upper... so i dunno if you'd really call this a 'kit' upper... but in any case, this whole issue about the higher pressures in the carbines must be why they sell 'enhanced bolt /w dual extractor springs' for an extra $100ish (which i obviously didn't get). |
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slayer_taunu, To bring you up to speed, when the bolt starts to unlock, the spent case is pressure bound in the chamber, and the bolt/extractor slips around on the case rim until the time that the barrel/bolt lugs clear each other and the bolt starts to pull the pressure bound case out of the chamber (read spent case is not rotating with the bolt). During this slip/rotation is when the extractor is shaving the brass. So with that in mind, lets let the rest of the rifle break-in by live fire, but the extractor needs a little TLC. With just the extractor in hand, use a jeweler’s file and de-bur the end of ammo case rim relief channel edges. Next hit the outer flat edges of claw (where it grabs the case rim) to break the end burs. If you have a steel counter sink, hit both pin entry points to break any burs here as well (this just makes sure that the extractor is not binding on the pin.). Also, if your feeling up to the challenge, you may want check the bolt face ejector hole as well. This requires that you pull the ejector (read punch the roll pin out with a 1/32" punch) and run you finger nail over the bolt face across the hole. If the bolt face hole edge has a burr/crowned (raised rim section when it was drilled), you use a stone to flush/surface the opening with the bolt face. Also, while you have the bolt in hand, check the rest of the bolt face, you may have a bur at the edge of the extractor-bottoming pad as well. If you need a few photos of what you doing, IM me with an email address and I can send you a few shots with a tool pointing at the areas. P.S. I consider anything less than a “Complete built rifle that has been test fired with all of the end product components” as a kit. So, yes even if you started with a factory rifle, and installed one single different part, you just became the smith and need to check your own work. |
I shot my first build this weekend and had the same trouble but mine did it from the very first shot. This was quite baffling to me considering that when I close the bolt and tried to manually extract the casing it would slip right off, but with a a good bump to the stock the empty case would fall out  . There is absolutely no damage to the spent brass so I guess the thing to do would be to give the o-ring a try. What a freaking bummer today was though.
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i just took mine back to the range yesterday... put 30 rounds through it with no problem. had a few bad extractions in the next mag. i pulled those rounds out, and didn't have any more problems. i did dissasemble the bolt at the range and didnt find any brass stuck in it, so i'm going to have to attribute the issue to 'chamber needs to break in' as stated in one of the previous posts. I may also changing my buffer... can buffers/springs wear out? my lower/stock is a pre-ban, so i'm sure it's seen some use before i got my hands on it. |
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The only thing that will wear out on the buffer is bumper end (read fall apart after getting rock hard). The buffer/recoil spring on the other hand does goes soft, and changing it may be a good idea if you don't have a idea of how many rounds the rifle/spring has seen. When the spring goes soft, you start to get overaction (B/C on the way back), and fails to strip a round out of the mag (B/C on the way forward). |
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