The key is picking the right bullet hardness for the right velocity.
Too soft will lead because it will smear through the rifling instead of biting into it and spinning. Too hard will lead up as well, though. This is because if it's too hard and doesn't fit into the bore properly, some hot gases will pass *around* the bullet and case some lead to melt.
Basically, slug the barrel and use the correct diameter bullet driven at an appropriate velocity and all will be well!