I have a S&W 40 M&P Shield. A friend of mine gave me a bunch of his extra reloads he made. I have probably put about 200+ of these thru this gun. I have noticed build up between the lands of the rifling. I have tried just wetting it with CLP then using a brush to clean that gunk out. I got most of it, but then I went out again and noticed the build up again. I want to soak just the barrel in CLP for a few hours or longer. Will this affect it negatively in any way. I have tried bore snake, brushes but never a long soak. I have a feeling he used some crappy powder that leaves a lot of residue. I have about 150 rounds left. I really don't want to use it if you guys think that's the cause of this build up. BTW-It smokes quite a bit when firing. I need some help with this problem.