Did you look down the bore using a light at the other end . Look down the bore with a bore light . Wal-mart has a nice winchester one for $5 . If the bore walls are still shiny smooth with no dull areas or pitting it was probably not rust .
After just writing the above I looked up KG 12 . It is a water based bore cleaner and you left it in your bore for a long period of time
What is a long period of time 20 min or 2 months ? 20 min should be no big deal but if you ran a wet patch down the bore and then put the gun up for a month . that may not be good .