If you get on eBay and buy one of the old Lyman shotshell reloading guides, they were very good. Some of the components on there are not available anymore, and of course some of the new stuff isn't there. But it is still useful in many cases. There won't be any data in there for loads with buffer. But you can use lead or copper plated or whatever kind of shot.
Loading buckshot and slugs is kind of a specialty thing. There is one company that specializes in that. Forgot the name but you can find it online. The only problem or challenge there is they're load data is not open source, and much of it is specific to their particular components.
Given that buck shot is $2 a piece, it's probably becoming worthwhile to look at loading that.