Judging by the number of threads asking for help troubleshooting an empty case and live round stuck in an AR action (often mistakenly called a "double feed") it's safe to say it's a common malfunction. It is also one of the most misdiagnosed problems. As soon as someone posts to ask what cause this, folks start going on about what kind of ammo is being used or suggest heavier buffers & new action springs and trying a different magazine. Others will blame it on over-gassing or short stroking (I've been guilty of the latter a couple times myself). But none of these things will fix the real problem- weak extraction/ejection. Usually the problem is weak extraction.
First, let's talk about the most common misdiagnosis and get it out of the way- Over-gassing leading to the extractor jumping the case rim. If over-gassing is causing the extractor to jump the rim, the extractor and/or spring is failing. If the extractor & spring were both good, the extractor would tear the rim before jumping it. I've seen this first hand when pogo-ing the rifle to remove stuck cases from the chamber. If the case won't come out, the extractor will tear the rim off. While tuning an over-gassed AR will often eliminate the extractor jumping the rim, it's just a matter of time before extraction fails completely. The violence of an over-gassed action just reveals extraction problems sooner. So really, changing buffers, action springs or ammo won't fix the problem, they will only hide it.
Neither will short stroking cause this problem. I thought the empty/live round malfunction of a shortie I was trying to get dialed in was due to short stroking. This misdiagnosis was making me crazy, as I kept trying to tune the adjustable gas block for reliable ejection with softer recoil. Turned out the real problem was a flat extractor spring. The bolt was replaced and suddenly, tuning became a snap.
If the malfunction is due to short stroking and the extractor & ejector are in good shape, the case will eject long before the BCG travels rearward enough to push the next round out of the mag. The empty will eject when the BCG is about halfway to the rear. The bolt will have to travel almost all the way to the rim before it can enough grip to push the case. If the rifle suffers a failure to feed from short stroking, the empty will be ejected.
Next time someone asks about their AR jamming with an empty/live round caught in the action, remember, it's not from over-gassing, short stroking, ammo, action spring or buffer. It's either the extractor or the ejector. That doesn't mean the rifle doesn;t have other problems. But those won't be easy to find & fix until the extraction/ejection issues are solved.
One last thing- Nearly every rifle I've seen that suffered from this malfunction was due to weak extraction, so check that first
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