flash-rusting is just as it sounds. When using boiling or exceptionally hot water to clean/flush the gun, then drying the bore, by the time you grab the oil and another dry patch, the bore is rusted one end to the other. That is flash rusting. it happened to the TC I bought in 73, Taylor's TC from 69 to 74 or so, and Peter's .40 longrifle that Taylor built him. We stopped using hot water then, however Pete didn't and over a numbe rof years, ended up with a pitted bore, one end to the other.
Two lead-lappings later, the bore of Peter's rifle is now usable, however it will tend to foul a bit - not enough to make one wipe the bore, but more-so than our other guns which have never flash-rusted.
Red beads or streaks are found after firing, not after cleaning.