I'd bet money it's a saami chamber. My hunch is you're not pushing hard enough on the shaft of the Hornady tool. Which by the way, isn't a super accurate way to measure base to lands but it will get you in the neighborhood. How can I prove that, you might ask? Give that tool to 3 different people and you'll get 3 different final answers. They might be close but they won't be the same.
By all means, do what your smith has said because he's not been wrong yet. But what I'd do to prove the whole carbon ring...chamber one of your factory loads and see if there are scratches on the bearing surface or land marks on the bullet. If there is nothing...push harder on your Hornady tool. If it's a carbon ring you'll have scratches on the bullet bearing surface and it will be hard to chamber.
You should, at least initially, be pushing hard enough on that bullet in the Hornady tool that you're pushing the bullet into the lands and when you pull the tool out, the bullet is stuck in the barrel. Use a cleaning rod to gently push the bullet back into the action. Closely paying attention not to nick the crown or bang up the inside of the barrel with the rod. Bore Tech makes a great bullet pusher jag to go in a rod.