The easiest way to remove it is to use a metal rod (old cleaning rod) that is slightly smaller than the bore diameter. Insert it through the muzzle and tap the BB out. I would put a little PB Blaster or other rust remover in the bore first and let it soak for a few minutes. If you don't have any rust remover, a few drops of gun oil or 3 in 1 will work.
Let me know how you make out with it.