here are a couple of things that come to mind, considering that it is a new gun did you scrub it good with some thing to remove the factory barrel presevitive? [ like breke cleaner] it can look like that if you left some. If you are using 777 I have had it leave a brownish residue that I thought might be rust startingtill I could not find any evidence of rust after I scrubbed it out. also if you are using copper plated bullets like the Powerbelts make sure it is not copper rub off. Lee