From the Book:
Barrels have more than the rifled bore itself. Most people neglect to clean a very important part of that barrel: the chamber. This is of particular import to semiautomatic firearms because the cleanliness of the chamber is vital to the proper functioning of the gun. The common thing that happens is that powder fouling and residue, brass shavings from the case, general unpredictable debris like sand, and even the junk that some companies use to rust-proof steel cartridge cases will make its way into the chamber. This isn’t even counting leftover machining issues that came from the factory, such as rings, bulges, and scratches from the manufacturing process. You can’t really do anything about that stuff and, until everything else is eliminated from possibility, don’t worry about that kind of chamber damage; and if you do, take it to a gunsmith to have them investigate and repair.