When you shot the Hodgdon and IMR powders, was it in freezing temps?
The bore of any gun if taken from inside to out and back in cold weather will sweat, freeze, and sublimate. This explains the mid bore rusting. Air can circulate some from the action opening and the barrel ends. I ran into this problem with my muzzleloader. It caused several problems and misfires during that hunting trip, and will not happen again, If I am going to hunt in freezing temps, I won't load my gun until it's at outside temperatures, and I will also leave it outside if keeping it loaded for more than one day.