A letter from a federal firearms agent urging a gun manufacturer to help track the illegal use of its weapons has become crucial evidence in a lawsuit by 12 California cities and counties against the gun industry. The suit is based on a novel legal theory, and the gun industry has a long record of winning cases. Lawrence G. Keane, general counsel for the National Shooting Sports Foundation, the gun industry trade association, dismissed the letter. "I am not concerned about any document that is going to come out," Mr. Keane said. "There is nothing that the firearms industry is trying to hide."