Homer, I've cast a lot of grooved lubricated bullets with straight pure lead and got perfectly filled out bullets. I do find you have to run the heat a bit higher than when casting round balls with pure lead.

Even round balls of pure lead must be run considerably hotter than about any alloyed lead.

The addition of a bit of tin - say 40:1 lead:tin will still be quite soft and should expand OK at sub-sonic speeds. A flat nose with help.

You can go even softer, with just a tich of tin to help fill out the mould.

With powder coating there should be 0 leading up to about 1,400fps.