RCBS used to proof their moulds with 1-10 alloy, that is 1 part tin to 10 parts of pure lead. It would seem to me that this would make the bullets heavier, not lighter. If weight is your only criteria, try casting with pure lino, it should get the weight down an appreciable degree. You could also cast with pewter to lower the weight as well, but that might take you a little to far to the light side.
