Have a new 870 exp that I purchased from a neighbor. He mistakenly bought an express when he really wanted the 870 that would receive 3.5" shells. Claims he never fired the gun, but jammed the gun trying to load a 3.5"shell. Everything on the gun seemed to be fined until I tried to load some shells in it. The action will not lock when closed (w shell). When it is empty, it closes & locks fine. Suggestions?