Educate me on this infamous thing called a Carbon Ring.

[SouthPaw]

Absolutely possible, but unless I'm misunderstanding something, that wouldn't explain the sudden pressure signs and bullet scoring.

Additional friction due to a tight freebore certainly contributes to more pressure, especially as your barrel is "speeding up" (caused by the barrel roughing and building more pressure anyways). With no tolerance in the freebore, it scores the bullet as you chamber it with any kind of minimal buildup in that area.

I had the exact same thing happen last year on a 6cm barrel. It was a prefit from a vendor that is popular here that was purchased during an annual sale. Likely lots of barrels being chambered by that reamer back to back. 200 rounds in, with loads below book max. Pressure signs, resistance when closing bolt, marred bullets... Sent the barrel back and sure enough the chamber was out of spec at freebore. They put a fresh reamer in it and voila no problems now.