To harden the lead for centrefire ammunition you need to add tin.
If you have water pipes with soldered joints melt them into it. If you collect range lead you will find most cast bullets already have some in so add this to the melt as well.

When I first started casting for my .357" revolver I used to heap loads of tin in the melt thinking this was the way to do it. As an experiment I just added small amounts of tin until the cast ingots/bullets had a slightly frosty appearance and these worked just as good as the bullets I had cast with a high tin content.

I have now got a .357" rifle and I use the same method to cast the bullets and they are accurate out to 100 yards.

Later roofing lead contains zinc, you only find this out when you melt it, as tac said.