I have a used 12 ga.M1, but in new condition, that has a problem with the shell feeding onto the carrier arm and thus causing cycling problems. When investigating the problem by manually feeding the shell onto the carrier arm by pushing the cartridge drop lever and feeding a shell from the magazine tube onto the carrier arm the shell doesn't feed completely onto the carrier arm. However, I noticed that when the end cap isn't screwed onto the magazine tube the shell feeds correctly onto the carrier arm. If the end cap is screwed only lightly the shell still doesn't feed correctly. What can be done to fix this problem?