Then order yourself a Teslong borescope camera. Cheap and you'll appreciate being able to see inside the barrel and chamber.
Problem solved - my teslong borescope came today and I gave things a look. The action screw was just the tiniest bit proud - I had checked with a flash light earlier and it had looked normal, but the the extra light of the teslong and the ability to look at it from multiple angles proved it was a bit proud. I then looked again at the lugs under a bright light and there was a very tiny amount of scoring about where the action screw would touch. Not enough to raise my attention earlier but now seemed to fit with the other info. I had put the 12FV in a new B&C stock before shooting it and it looks like that switch resulted in the slightest of protrusions of the action screw into the action bolt opening itself. A slight grinding of the end of the action screw (and re-bluing of the bare metal end) put everything back in working order. The "shortish" un-fired Hornady cycled like before, Hornady once fired that was a little tight on extraction was back to normal and the un-fired Nosler did allow the bolt to close even if still a little snug. I FL sized a new piece of Nosler 6.5 brass and it worked just like the Hornady. Problem solved. The Teslong has already paid for itself.