The heated barrel causes moisture( water) in the barrel to evaporate so fast that oxidation occurs.( rust!) That is why we call if " Flash rust ". It happens so quickly it can't be stopped fast enough. Use tepid or even cold water when cleaning your barrel, and you will have time to dry it with dry cleaning patches, and then lube the barrel after it is dry.
A new formula is showing great promise in guns, and it consists of one part WD40, one part Mobile One Jet engine oil( LP2), and two parts beeswax. This makes both a preservative to prevent rusting, and a great patch lube. The revolver shooters are finding that if they use this mix to lube the mouth of the chambers after seating their RB in each chamber, that their groups are dropping to half the size they get using anything else.
I have not yet found this synthetic oil in a store, here, locally, but still have some places to check. Most of us have WD40 in a spray format, but need to buy it in a can. The mix goes on patches similar to Dutch Schoultz Dry Lube forumla, as the driers in WD 40 evaporate and leave behind an oil compound in the cloth patch, and help to spread evenly the wax and synthetic oil before evaporating.
This mixture was developed by the same man who found that Teflon made a good lube for ML rifle patches. This formula is the only combination of ingredients that do as well as a patch as does teflon products, according to his own research.
It might be worth giving it a try. I want to try some on some metal to see how long it keeps rust away. Then I will try it as a patch lube to see how well it shoots.
Paul