IC or modified will work. Try both to see which is more accurate with a particular load. My mossberg was more accurate with the modified choke in it. DO NOT shoot your screw in choke gun without a choke in it. They were not designed to be shot this way, especially with slugs. Probably says it in big red letters right in the manual.

You can shoot most rifled slugs with a full choke, but they usually are not that accurate. Anything more than a full choke I wouldn't do it.

Paul