Most military shotguns that are intended to shoot 00 buck are a fixed cyl. choke. Mine shoots large buckshot and slugs best with a Imp. Cyl. I was always told to never shoot a slug out of anything tighter than a Modified.
All of my factory Mossberg full and X full chokes say "No Steel Shot" right on them. Steel is normally shot with a Cyl-Mod. choke. I have one Hastings Mod. that is specifically marked for steel shot.