Yeah, I've heard of it and experienced it. I think your Gun Smith is on the right track. I would chuck 0000 steal wool, saturate it with oil and spin it in the chamber.
Same, with the same solution. If it has more than just minor tool marks, I'll gently lap the chamber with a piece of fired brass from that barrel, using fine lapping compound on the case body (not the shoulder, that changes headspace). Then finish off with the 0000 steel wool to burnish it.
