RE: Bore Butter causes rust??
Natural Lube 1000 (bore butter) is, and has been, an outstanding lube for me in many TC Hawkens for many years and is the only thing I have and will use.
And I would agree if a bore is not properly dried that rust could form...but that's not any fault of the bore butter.
After a 100% hot soapy water cleaning & bone drying the bore, I pack bore butter very heavily...very heavily...into the bore and leave it there until the next time I go to the range or to hunt, dry patch out the excess and enjoy...worked perfectly for almost 15 years now.