I melted down some old pewter cups to get tin for adding to my lead mix for casting. Pewter is apparently 85-99% tin, with the rest mostly antimony and a few other trace metals but no lead. I poured the pewter into an ingot and it poured very well and clean.
The addition of a small amount of tin to a lead casting mix helps with the flow of the lead to fill out the mould so on it's own I imagine tin should cast bullets very well, although a little expensive if casting bullets in bulk.