The short answer is, you need a bunch more information before you embark on this endeavor.
Bird shot is between 2 and 9% Antimony. For a slug, you want less than 1% antimony. Antimony is a hardening agent for lead. Slugs need to be soft to seal properly and shoot well. My preferred alloy is 20-1 lead to tin for slugs. Tin will harden lead a little but it makes it pour better and fill out the mould properly.
I have loaded shotshells for nearly 35 years (yeah, I'm, getting old). I just started on the slugs this year. I have cast bullets for 26 years and just started casting slugs this year.
You have quite a learning curve ahead of you before you get into this. Purchase "The Cast Bullet Handbook" and read it cover to cover before you consider getting started.
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"Robo is right" Fuzzball