Greetings, so you want to load slugs then get to know your pressure sign's and even tho some will say there are no pressure sign's in shot gun, watch out for any pressure signs like Ext marks or primer flattening etc:
those are clear Federal hull above and
Fiocchi hulls below in the picture:
The Fiocchi hulls at least are known devils as we already know they are made a bit softer and show pressure signs early on, so we use that knowledge to our advantage a use those known devil hull in our tests than some unknown devil hulls! ( I mean in a good way )
Once I have my loads developed then I switch over to Federal hulls as they are made much stronger and not always show pressure signs!
Your safety comes first, so always play it safe!
Best regards,
Ajay Madan