i have been using bore butter from 1971 to now.no rust i ever saw but i clean after every shooting with water/soap.i did get brown patchs all those years .NOW, i only use bore butter with roundballs.why, i makes it easier to push ball in and fouling is reduced. i clean after 5 th shot as my rb get hard to push in.my patchs are bb.i stopped using maxi-balls for hunting and now using powerbelts,so i dont need the bore butter for hunting and i use clean barrel.95 % of my shooting is rb at range. 1 week before deer season, i load up the powerbelts and sight in. takes about 6 shots as they are not cheap.then, after season, back to rb and bore butter. for storing i use sheaths, no more bb even tho i never had any rust that i could see with bb in 35 years.