High humidity is the worse propagator of rust in barrels. As to cleaning, we've been doing it the same since we started- barrel ALWAYS comes off the gun and gets stuck into a container of water. The barrel is cleaned by pumping water into and out of the barrel using a rod and jag with doubled flannelette cloth. They are then dried, sprayed with WD40, wiped out the excess then stored muzzle down - no further oils or prep, even for long time storage- NEVER any rust. I assume in both these instances of rust with Ballistol, there was no WD40 treatment after drying.