Sometimes guns are so badly fouled it takes longer than that, even with the correct solvent. Even scrubbing the bore for days doesn’t ensure the rifle is clean, because if you’re using the wrong solvent, it accomplishes nothing.
That’s the equivalent of running your car through a puddle and claiming you washed it. Many shooters run a few patches through the bore, maybe make a pass or two with a brush, and assume that they have cleaned the rifle. The truth is that while many people think their gun is clean, it really isn’t. When that shooter comes to pick up his gun, I smile, hand them back a rifle that is shooting well again, and let them think I am a miracle worker. I can’t begin to tell you how often my cleaning alone will fix the problem. So, I’ll take the rifle, and my first step is to clean the “already-cleaned” bore, and test-shoot the rifle. Many will insist to the point of confrontation that they “already cleaned it.”
I have a checklist of steps to correct that, but my first step is to ask the gun owner a question: I’m the gun guy in the neighborhood, and my gunsmith shop is the first stop when somebody is having gun troubles-often an accurate rifle that suddenly started misbehaving and spraying the target with patterns instead of tight groups.