Bore solvent is a liquid cleaner which dissolves away powder and other deposits from the rifling of the barrel, the action, or any other place on a gun when buildup occurs. Solvents vary in efficacy and the specific substances they'll dissolve beyond copper, lead, and powder residue, but most people use Hoppes Number 9 solvent. I like to dip my cleaning brushes in solvent before running them down the barrel.
Bear in mind that copper solvent will dissolve brass brushes, as brass is an alloy of copper and zinc.
-- AdamSmith - 26 Feb 2006