Some use boiling hot water or hot water. As RC said, hot water will cause flash rust. For some reason, some don’t seem to experience flash rust but I sure did when I first started in muzzleloading and it was suggested I use hot water. If you’re using hot water, that just may be where your rust is coming from. Secondly, if the bore is not completely dry before oiling, it will rust. I use tepid water to keep flash rust at bay and dry out residual water with a little isopropyl alcohol and a couple dry patches.
You probably already know this, but you will need to follow with a good gun oil (which you should remove prior to shooting). I see a couple folks recommended barricade, but any good gun oil will do for a few weeks storage. Some are more picky about gun oil, and I would be too if I was storing guns for a LONG term. I have a couple guns stored with breakfree CLP that I check every few months and they have no signs of rust.