The foster type slugs with the "rifling" on them are actually swaging lines on the lead to allow it to swage down going through tight chokes, they will make contact in a standard cylinder bore barrel usually, but will reduce down when forcing through chokes. If you use choke tubes you need to be absolutely sure they are tight as it is possible to tar them out of the threads in the end of the barrel.
My old 1100 20ga has a mod choke in it and will keep 2.5-3" groups at 50 yards with standard Remington slugs. Acceptable for a smoothbore shotgun and the distances where I usually hunt.