I don't even use hot tap water any longer. Ambient temperature water seems to do just as well.

I had used boiling hot water to rinse the barrel with. The barrel was almost too hot to handle. This was in an effort to get it to dry quickly. That probably had something to do with it. I did get the rust out, after soaking the whole barrel overnight in an EVAPO-Rust bath. No more rust, but no more blue either. That's OK though. I wasn't really satisfied with the blue job anyway. I left it in the white so if it happens again I can just swab the bore with EVAPO-RUST and let it dry in the bore before putting a coat of oil in it for storage.

Did you blue the inside of the bore with a cold bluing solution? If so, you pretty much guaranteed rusty patches after a hot water bath.

No,LOL. To blue the bore of a barrel has never entered my mind. I was putting the gun back together a little while ago and got down to putting the last 2 parts on, the nipple and clean out screw. I thought After removing them I had placed them both in a piece of tape and taped them together, then placed them with the other parts in a zip lock sandwich baggie. The clean out screw wasn't in the tape with the nipple. I looked everywhere around the area where the rifle was taken apart. It was nowhere to be found. Only one place left to look, the trash can. Digging deeper and deeper and there it was, inside a piece of tape like the nipple was. All is well.

+1

i know what flash rust is and thats not it. The bores of my guns are not cleaned with hot water or any water at all. Some of my muzzleloaders go for months without being fired. The brown discoloration is on the patch when the bore is swabbed prior to loading.