Sounds to me like your friend had not really cleaned that barrel well for awhile and you had to work to get it clean!

I too use boiling water. There is no way you are going to remove every bit of water with a dry patch. But if the barrel is very hot, those spots will evaporate quickly. I lean mine against on outside wall, pour boiling water down it, then immediately run dry patches down it (while holding it with an oven mitt).