I have used the JB for years. Yes it will turn black at the blink of a hat. After u make a 100 or so strokes run a cotton ball thru the bore. If you see any cotton snagged in the bore try another 100 strokes and try the cotton again. From then on use 50 strokes and continue until u do not have any cotton hanging in thr bore. it will slick it up!! I have a 220 swift heavy barrel varmit rifle that picked up 180 fps after I polished the bore.