Just clean it the way you normally clean. Solvent or CLP. But make sure it's only a stain not a brass shavings that's been accumulated. If it doesn't come off and really bothers you that much, try to put little dap of Sweet's 7.62 solvent on the stain. Make sure it doesn't smudge off to surroundings. Leave it on for 3 minutes and wipe it off with other solvent.