You could also try soaking them in sulfamic acid, which is how electroplaters clean lead before copper plating it. Sulfamic acid can sometimes be found at flooring places, as it is used to removed excess grout after laying tiles.
Whatever route you take, consider that removing lead oxides will produce lead compounds that are either soluble or finely divided, making them easy to ingest. In other words, the perfect kind of lead to cause lead poisoning. And then weigh that against the fact that oxides on your bullets cause no harm at all, it is strictly cosmetic.