I have a Mossberg 402 lever gun that shoots 22s-l-lr. It is a 1964-1971. The problem is when I close the lever, it drops down or pops open a bit. I know it's not supposed to do this and I'm not real familiar with the inner workings of lever actions. What could cause this and how can I fix it? Thanks in advance and I apologize if this has been discussed. I tried to find it but was unable to.