The head of the case must not be flush with the chamber mouth, so the action won't close. Check to make sure the ejector or extractor is slipping over the cartridge as the cartridge is inserted in the chamber. I'd do that with the barrel removed from the frame. If it has an ejector or an extractor, you should be able to slide the cartridge into the chamber and the ejector/extractor will move away from the cartridge and slip in behind the rim. The head of the case should be pretty much flush with or slightly below the opening to the chamber, it it's sticking out any that you can see, it won't close. If there's not some factory crud in the chamber, it must not have been cut right.