Pyrodex is corrosive. Some say it is more corrosive than real black powder. IMHO 777 is also corrosive. Maybe not as much as Pyrodex, but still could damage your MZ, if not properly cleaned after a shooting session. I would not risk damaging my MZ's by not cleaning after a shooting session. I have used both of these powders, as well as Goex. I have never had any corrosion issues as I always clean after shooting.