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For many years, "hard cast" was defined as anything harder than a bhn of about 10. People think their cast bullets have to be super hard to avoid leading the barrel. My "target" hardness for use in the .357mag and .44mag is 12-14 for full power, high velocity loads (up to about 1450fps). Anything harder than 14 tends to lead the barrel as does anything softer than 12 at that stated velocity.
The reason the harder cast bullets will lead the barrel is that the lead does not obturate (expand) to seal up the barrel and kind of skids down the barrel as it starts to spin from the rifling.
When shooting cast bullets, the saying "Fit is king" has a lot of merit. You typically want the bullets to be sized to .001-.0015 over your groove diameter. Some guns prefer a little less, others prefer a little more. Combine "proper" fit with a good lube and you can be shooting bullets with a bhn of 10 up to close to 1400fps without any leading.
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