Be sure to use a good powder solvent (Hoppes #9and Butch's bore shine are 2 good ones). I dip a bronze brush in solvent and work the bore, let it sit for a few minutes (or overnight) follow with a few patches soaked in solvent, then dry ones until clean. End up with patches soaked in rust preventative (CLP, Kroil, or rem oil are good ones). Repeat above if needed. Good luck! BTW my Mossberg had some sort of brown rust inhibitor in the bore when got it, shooting it baked it in. It took lots of elbow grease and Butch's to cut thru it.