While attending the Infantry Officer's Basic Course in 1992, we had a hose connected to a hot water source that we used for initial cleaning of M16A2 uppers and lowers. The water was so hot you could barely stand coming in contact with it, and it got the metal so hot you could barely touch it afterwards, but it tended to evaporate quickly!
Seems we never did this when the Platoon Trainer was around, so it was like sneaking off and doing something the "short cut" way, but I'm sure the cadre all knew we were doing it. and of course, we finished the cleaning and lubricating in the proper way. the hot water just got the big stuff off quickly.