The lead used in lead shot varies in amount of inorganic arsenic depending on the size of the shot. Contact Metallico Granite City and ask for a materials sheet on their shot. At 25 cents a pound it's worth messing with, but only cast in a well ventilated area, as the arsenic (used to reduce surface tension of the lead when making drop shot) is bad stuff. There is very little tin, so the addition of some 50/50 bar solder will help your moulds fill out a lot better. I casted some 225gr (pure lead) bullets and found that with the shot lead my bullets were about 216 grain and were too hard to mark with a thumbnail. My accuracy was tolerable, but leading ahead of the chamber was too much for me.
Where in WI are you?