For anyone still interested in this topic...I have a friend who is a gunsmith and I just took it to him. He did quite a bit of testing with it to confirm what he thought it was. He says that at some point, Mossberg changed the spring in the bolt that operates or has something to do with the firing pin. He says the old ones were "looser" and that the new ones are often too stiff. He put in an old one that he had and it has worked fine since.