I have found that harder cast lead is ok. Best is coated lead.
However if you have an alternative then I would try it. Unless you are shooting thousands of rounds without cleaning then you should be ok. As a precaution run a bore brush and patch every hundred rounds or so, and do not shoot jacketed until you have thoroughly cleaned the barrel.
Most people why say that you should not have no real experience shooting lead through polygonal barrels, but have been told (or read on forums) by other people who have been told (or read on forums) that it is dangerous.
Personally I have never seen nor heard of a kboom directly releated to lead. I think that people see a kboom due to a badly loaded round, and so since lead is associated with reloading + manufacturers warning about lead they assume that it is deadly.
I do not believe this is the case unless you are shooting huge volumes without cleaning. Like Maurice said it is a ##### to clean, so if you can afford to use coated lead projectiles then that is better. I use coated projectiles because it is easier to clean the barrel afterwards and you don't get that unpleasant (and unhealthy) lead haze after shooting.