You would have to set back at least 1/2 " to clean that up. A fellow gunsmith I knew, who was a very skilled workman, had a reamer pick up a chip and ruin the chamber. He bored the chamber out, up to the shoulder, then made an insert which was a shrink fit into the barrel and chambered that. When the owner wore that barrel out (it was a 6-284, so it didn't take long) the 'smith cut it apart and showed him what he had done. An impressive bit of machining. When I had a similar thing happen, I just bought a new barrel and replaced it (this was when Shilen barrels were 150 bucks). I then set back the messed-up barrel and it is still on one of my Model 70's. WH