Interestingly the hot water method mentioned in the OP was used in WWII, in fact guys would sometimes take the weapon into the shower with them. I have that on the authority of some old boys who had been there and done that. But, and its a big but, after the weapons were cleaned and dried they were lubricated. Apparently somewhere over the years the lubricating part was forgotten by some folks.

When I was in the Army small unit armorers all had some level of competence. Usually in small units like the one I was in in Korea (a detached company) a soldier who knew something about firearms and firearms maintenance was given the job as an "other duty as assigned." For reasons unknown to me, in Korea, even though I wasn't the armorer or the machine gunner the powers that be decided that I should be the one to take care of the M60s so after a crash course on "the pig" I became the person who had to check them out before issue (which was seldom.)