Lead hardness 00 buckshot

I see on the outside of my Lee 00 Buckshot mold "pure lead should be used for round balls" is this a hard and fast rule? has anyone used variations in hardness? I've a shitload of pure lead so no issue there but just curious! will start casting in a few days :)

You could use harder lead but pure lead will expand better, if you have trouble with the buckshot not forming proper round balls add a little tin to the mix it breaks the surface tension and the lead flows
into the mould better.