Just out of curiosity... were these blued barrels or stainless...
They are blued barrels.
i've always left my guns loaded for extended periods of time. Never had a problem when using Pyrodex or Triple Seven. The corrosive effect of the potassium perchlorate in those powders may be mitigated by the sodium benzoate.
IMO: Black MZ must contain a huge quantity of potassium perchlorate.