I had two that wouldn't eject. I messed with them until I figured out what the difference was. When I finally figured it out, and corrected mine, it never bobbled again.
So, are you gonna tell us or do we have to pay for your findings?
I've gotten far too much good free info here to ever ask for renummeration for information. I am reluctant only because, until I get more information, I won't know if it is the same problem.
That said, the problem with mine, in both cases, was a carrier that was too high in the rest position. If an 870'is running right, it will "usually" eject nicely even if you run the action as slow as you can. When the action closes, when the bolt stops, the action bars still must move forward a bit more. This motion operates the part of the bolt that rises up and locks into the barrel extension. So, when you retract the bolt about halfway, you will see that you can hold the slide still and push the bolt backwards a bit. In operation, as you draw the bolt back, at some point the mechanism releases the next shell. This shell is under magazine spring force, so when it is released it is "launched." It slides up the carrier, hits the bolt, and knocks it back that fraction of an inch. If things are as they should be the bolt is in the right spot that when it is knocked back the empty is ejected. It is not you running the slide quickly at all. Again, if it is working right, you can run the action at literally a snail's pace. Even then, when that incoming shell releases, it will hit the bolt and kick the empty.
However, if the carrier rides too high, there is not enough room for the incoming loaded shell and the waiting to be ejected hull. The hull heads interfere with each other and the empty doesn't get a clean exit. It is difficult to impossible to run the action fast enough to prevent this. This is because that incoming round is released before you draw the bolt back far enough for that motion to eject the empty (as with the last round in the gun, when there is no incoming loaded round to hit the bolt.)
Some people tell me you can bend the carrier down. That looks problematic to me. I remove the trigger group, and file a bit on the surface that determines where the carrier rests in the "ready" position. It is a cut and try thing. You are trying to drop the tip of the carrier, but you don't want it sticking out way below the bottom of the receiver.
I hope my description makes sense. All that said, I'm not an 870 expert by any means, and I'm not a gunsmith. I am an engineer and I'm pretty good at figuring out how things work. I can only relate was has worked for me in only two cases.
