I have a friend who has a Mossberg model 500E 410. He say's that sometimes it fires and sometimes it doesn't. I brought it home and took it apart and looked at it and there appeared to be nothing wrong, so I went out and shot three shells with no problems. Gave iit back to him and the following day he said he had the same problem. I"am thinking that the firing pin is to short. Any other ideas as to what the problem could be?