What caused this? New photo

[mauser284]

It is a scratch in the chamber. Who knows how it got there? Who knows if it left the factory that way or was done by previous owner? The brass will always reflect the metal surfaces it is expanding into when fired. Repairing a chamber is not a modification if you car breaks and needs a part replaced to function like it did when new that is maintenance not modification! Polishing the chamber will not make your cases stick if you use the proper grit. People use too fine of a grit and do too much. If it does not catch a finger nail it is not anything that needs to be worried about. Lets say you drop a round onto the ground pick it up and chamber it? If you did not clean it off before doing this any dirt on the round will now expand into the chamber and the brass and when you extract that round it will drag across the chamber scratching it. Since the line run North South not East West means it is more likely it was done by someone shooting the rifle after it left the factory. If it is dented that is another issue all together. I would check the chamber ASAP. I have seen too much lube, carbon build up, dirt call cause dents upon firing.