Lee Loader

Gonna start casting slugs for my shotgun (12 ga. Lyman 525 gr sabot slug). I have been casting bullets for quite a while and am familiar with alloying, casting methods, etc. but being a total rookie with anything shotgun, I'm wondering about slug alloy/BHN? I'm thinking slugs should be on the soft side, 9-12 BHN. Any thoughts?

Lee states pure lead, but I've used a Lyman #2 clone and my 6:4 WW/linotype mix. Mold needs to be plenty hot and smoked well so hollow base pins don't stick. I had the NOE Lyman Sabot Slug clone, but traded it away.