Leaving the solvent in overnight as John Traveler recommends is the normal solution. If it is a lead build up, get some lead wipe-a-way cloths, and cut a patch that will just wrap around your bronze brush. Scrub the area of the bore with that, it will come out black but that is O.K., just keep scrubbing until it is gone. If you are shooting lead bullets, make sure the velocity and the hardness of the bullet match, too soft a bullet will cause heavy leading very quickly. If it is lead, and it is really stubborn, get a Lewis Lead Remover, (a brass screen on a specially fitted rubber plug that scrapes the lead out without damaging the barrel).