Cast bullets, if made with real lead Wheel Weights, can be heat treated very easily. A lead/antimony home-mix will need the addition of lead shot, which contains the arsenic necessary to allow heat treating. WW already has the trace amounts of arsenic in them, which allows heat treating.

A simple lead and tin mix cannot be heat treated, without the addition of antimony and arsenic, I assume.

Heat treated WW bullets can easily exceed Linotype bullets in hardness by 50%, but without the brittle nature of Linotype.

The rate of heat treating is adjustable by the heat used, as well, temper can be drawn down if they come out too hard. Heat treating cast bullets is a science in itself.

You can Google Verl Smith for more details as he wrote the book on this subject.