Quoted:
With the higher pressure and very short dwell time of the short pistol barrels, the case never really has enough time to shrink back down before the extractor is trying to pull it from the chamber. Adding a O-ring around the spring increases the spring pressure allow the ejector to get a better bite on the rim. O-ring generally don't come installed so chances are there he cannot "go the other way and remove the o-ring".
The o-ring is done by NSWC-Crane on the short barreled CQBR and Mk-18 platforms to counter exactly these problems.
Right, but the extractor in the OP's gun is not having a problem getting the case out of the chamber. The gun is having a problem getting the spent case out of the gun and it's trying to jam it back into the chamber along with the next round. Is this also a common result of a weak extractor? I'm not arguing, just asking a serious question. It seems more like a problem with the ejector to me. I feel like people are stuck on the "failure to extract" problem when the gun is actually "failing to eject." I apologize if I'm off base here. As I said, I'm not an expert. I'm just trying to make sure the advice given is for the correct problem here at hand.
I had the behavior that the OP describes, and it was fixed by installing an new extractor spring and o-ring (I didn't have the o-ring originally).
It is hard to say exactly what is occurring, and failing to extract does also result in a failure to eject.
The spent case is coming out of the chamber at least partially, and may in fact be riding in the extractor until a new round dislodges it.
Why would the extractor have purchase on the spent case long enough to pull it out of the chamber, but then release it before ejection?
Could it have a weak grasp of the spent case, and the attempt at ejection causes it to be dislodged but not ejected?
If the extractor never had a grasp on the spent case, would the spent case get pushed out of the chamber by gas pressure?
Whatever was happening, stronger pressure on the extractor (due to the added o-ring) seemed to fix it. I used to get FTE every 3-4 rounds; since adding o-ring 500+ rounds without issue.