The rifle is short-stroking. Accordingly, it exhibits all manner of feeding and extraction problems.
[It may or may not be short stroking. Let's investigate a little further before we go down that road. Are you experiencing primarily the "classic" round over jam? This malfunction presents itself as a fired case failing to eject and being jammed up into the upper receiver by an unfired round being pushed forward by the bolt. Kind of a simultaneous failure to eject and failure to feed. If so, read summary below. If not, then please explain exact nature of the FTE and FTF.]
I've taken it apart fully, and found 3 things that I don't quite like, but nothing that appears to be obviously wrong:
1) The gas tube doesn't seal completely with the gas block. There's usually a bit of powder streaking visible after each trip to the range. Not ideal, but I've seen other ARs run just fine with this sort of "problem."
[This is normal in AR-10's. Look elsewhere for the problem.]
2) I've noticed that the hammer seems to have a rough edge which is beginning to score the bottom rear of the bolt carrier. "Aha!" I say to myself, and promptly replace the trigger group thinking this will solve my problems. No such luck. The rifle misbehaves exactly the same with the new trigger group as with the old one.
[Again, normal.]
3)The buffer spring is much stiffer than any AR-15 spring I've seen. But I've never seen another AR-10 buffer spring; maybe it's supposed to be that heavy in the 308. I dunno...
[Yes, the AR-10 buffer spring (operating spring) is much stiffer han that in an AR-15.]
My next course of action was going to be to replace the gas tube and buffer spring. But I've decided to seek professional help instead, before I spend any more money.
Summary:
As mpthole stated, many AR-10 problems can be traced back to the mags. Therefore, I suggest you use the 10-round mag that came with the gun for any future troubleshooting. Failing that, use a mag that is known to shoot well in another AR-10.
The "round over jam" is caused by the bolt/carrier travelling to the rear fast enough to cause the extractor to lose control of the spent casing. This results in the spent casing being extracted from the chamber, but since it is no longer controlled by the extractor, the ejector cannot act on it and it winds up bouncing around in the upper receiver. As the bolt/carrier continue to the rear and strip a fresh round from the magazine, this fresh round cannot successfully reach the chamber because it runs into the spent casing and jams.
The primary cause for this is a faulty extractor. However, there could be a number of faults. First, there is a tech note on the ArmaLite web site about a number of extractors that were supplied to ArmaLite which had been "improperly deburred." If you suspect this may be the case, send your old extractor back to ArmaLite, cite the Tech Note, and they will send you a new extractor free of charge. I did this, it did not solve the problem, so let's proceed with how to fix it.
1. Remove the extractor and assure you have the rubber o-ring installed over th extractor spring. If you do not, for test purposes, just put in a #60 O-ring you can get from your local hardware store. Not ideal for long term, but good for troubleshooting. ArmaLite sells the "proper" O-ring if you want to replace the test one later.
2. With 600 grit wet/dry sandpaper sand the edges(sides) of the extractor which contact the channel that it sits in on the bolt. In other words, wth the claw of the extractor facing down, smooth the long sides to the left and right and the curved rear surface. Burrs on the sides of the extractor can hang up (ever so slightly) on the bolt and cause the extractor to "pause" in its action during extraction.
3. Observe the claw of the extractor and assure that it is free of debris.
4. Re-install the extractor w/ O-ring. Using a dummy or fired case, fit the case rim into the bolt face forcing the extractor down so the case is flat against the bolt face. SLOWLY allow the case to rotate under the force of the ejector and observe any slight rubbing or loss of tension on the case by the extractor. There should be none. If you notice any, try to observe where the interference is and repeat step 2 above to eliminate it.
That should clean up the extractor.
A contributing factor to failure to extract could also be a dirty or rough chamber. In rifles, a polished/shiny chamber is a good thing. First, thoroughly clean your chamber with a good chamber brush. If you feel confident enough to try it, then polish the chamber with Flitz polish on a soft felt polishing wheel just slightly oversize for the chamber. The small round polishing wheels for a Dremel tool do this job well. If you use a Dremel, you will need the "extension" attachment to get into the chamber. Remember, easy does it. We are not trying to remove any metal, just polish it up a bit.
Put everything back together with lots of CLP and off to the range for some test firing.
One final note on mags. The only mags that work reliably for me are the new mag conversion bodies from ArmaLite with Wolff springs and the new ArmaLite "Mil-Spec" alloy follower. However, the main malfunction caused by "bad" mags was nosediving of rounds, NOT failure to eject.
Please let me know if any of this helps your problem.
