If the piece can be removed from the rifle, clean it to get any finger prints off of it, then tie a piece of string to it, pour some ammonia on to a piece of rag and place i a jar or sealable container, preferably one you can see throguh, hang the brass by the string in to the jar, close it up and let the gas do it's work, check after a couple hours, the longer it sits, the darker it gets, you can turn brass black simply by applying the ammonia directly to the brass, rinse with water when you are satisfied with the color.