Bench rest shooters use Kroil in combination with JB's bore cleaning compound. I use it before shooting any used rifle I buy. Clean the bore with Kroil and let it sit for at least an hour, then use the JB's, I use a bore mop, 10-20 passes through the bore. It takes some time and patches to remove the dirt, copper, and JB's, but you will have a clean bore. Bench rest shooters use this technique to re-juvinate a gun which seems to be losing accuracy.