870 Re-Assembly Problem

[10rounds]

Field stripped the 870 for the first time to clean. The 870 had been used a couple of times. On doing the function check, on an empty chamber, the shells can be loaded in the mag. However, when there's a shell loaded in the chamber the mag cannot be loaded because the carrier is partly blocking the mag port. The carrier doesn't want to go all the way up when there's a shell loaded in the chamber. What should I check to see what's stopping the carrier from moving correctly? The 870 was working properly before field stripping it.

[aippi]

Sounds like you don't have lock up. Reinstall the barrel and make sure the magazine cap is tight.

[10rounds]

Sounds like you don't have lock up. Reinstall the barrel and make sure the magazine cap is tight.

Re-assembled the shotgun several times with the same result. I noticed that with an empty chamber, the slide assembly is all the way forward. With a shell in the chamber, the side assembly is not all the way forward. It appears that the carrier's protruding fin is not meeting the slide assembly cutout when there's a shell in the chamber. I'll just take it to a gunsmith to get it fixed.