I recently spent some time researching this because a new Hell's Canyon was copper fouling, badly. Short of trying a long extended soak, which I didn't want to do, using a bronze brush w/ Kroil / JB was the only way to get it all removed. My groups tightened up immediately. I don't know, this one shoots far tighter when truly clean. With all the rifles I've recently tested, it's very rarely truly clean until you use a brush. Some will shoot as good or better if dirty, so I wouldn't use a brush there. In most cases, everything I've read (as credible sources as possible) suggest brushing your bore will not harm "the average" rifle and we are all just being overly cautious. Good posts N7XW.