these are JUST the projectile bullets, not loaded ammo right? Any sort of vibratory or other case cleaner may help - doesn't have to be sonic. I'm speculating on this but washing them in some brass or copper cleaner or similar might help too but I don't really know. Reloading shops used to sell liquid brass cleaner before vibrator units became popular - don't know if they still do. Make sure to get bullets totally dry afterward - set them in the furnace or boiler or other warm dry spot afterward.
HtH