If she was likely blocked up when purchased by your neighbor that didn't look things over first, she's probably already got lead splattered in the bore from the subsequent pokes. Try loading the bore up with PB Blaster or Liquid Wrench penetrating oil so that it can drain towards the muzzle and not the compression cylinder. Swab out any remaining liquid to prevent blow-back, seal the aft end with a piece of rubber inner tube and use your air compressor nozzle pushed up against the rubber seal to squirt the blockage out.
Plan B would be using a brass brazing rod to lightly tap said lead slug toward the shorter travel route of the lubed bore to see if it will budge at all.
May The Force be with you....
