Corrosive primers and back in the day caps had a lot to answer for, also cold damp climate and bringing cold kit indoors.If you prefer warm or hot water and have chlorinated water just bring it to a boil and let cool a bit, the chlorine will evaporate, it will do so if your water sets overnight also.When I was in the army we were made to use nearly boiling water to clean our M1 Garands. No flash rust but the rifle I was issued had a horrible dark and pited bore, I think some rust may have been beneficial.
Buzz