Re: 300 WSM feeding issue
Could be a number of things, before I started replacing parts I would check the magazine spring compression when loaded. Is the round that is not feeding always the first one, when the magazine is full? A common issue with any cartridge is when the magazine spring is fully compressed the top round (first to be fed) gets wedged between the follower and the action. There should always be some compression in the magazine spring when loaded. Check that first.
Good luck, Tony