I had a similar problem last year. At the end of the barrel extension is the ejector. If the ejector rivet comes loose, which is what happened in mine, the shell will partially eject and the fresh round will come up and jam. Take the barrel out and if the ejector is missing, then you know. In mine the ejector fell down into the gun and rattled around. Also, the suggestion regarding the action spring is good. There is an action spring follower that can break. If it breaks and shifts around, the timing of the gun is then off. Hope this helps.